Trends in Kinetic applications of the pulsed laser ... - Chemistry

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Trends in Kinetic applications of the pulsed laser ... - Chemistry

Trends in

Chemical Physics

Vol. 2, (1999)

Kinetic applications of the pulsed laser photolysis/ mass spectrometric

technique

J. Park and M. C. Lin

Department of Chemistry, Emory University, Atlanta, GA 30322 USA

ABSTRACT

The pulsed photolysis/mass spectrometric (PLP/MS)

technique has been applied to measure the absolute rate

constants and/or the product branching ratios of the

reactions of NH 2 + NO x , C 6 H 5 + C 6 H 5 , C 6 H 5 + H 2 /CH 4 ,

CH 3 + CH 3 and CH 3 + NO 2 using electron impact or

resonance enhanced multiphoton ionization. The

method effectively extends the kinetically useful

temperature range for these combustion processes to

~1200 K. The time-resolved concentration profiles of

the reactants and products allow us to determine

reproducibly the absolute values of rate constants as

well as the products branching ratios.

I. INTRODUCTION

Chemical kinetics, as in other branches of science,

is in need of a universal and reliable technique for

experimental data acquisition. Modern chemical

kinetics requires not only reliable reaction rate

constants over a wide range of temperature, but also

quantitative product branching ratios if the reactions of

interest have multiple product channels via different

reaction paths. The requirement becomes particularly

stringent and difficult to meet, for instance, if one is

dealing with an important elementary combustion

reaction with different product channels which become

competitive at elevated temperatures. Thus,

quantitative determination of a total rate constant with

product branching ratios for a reaction of interest over a

wide range of temperature and pressure is a serious

challenge to an experimental kineticist today.

Of all the established diagnostic techniques, which

have been adopted for chemical kinetic applications

including the very sensitive optical methods such as

LIF (laser induced fluorescence) [1], REMPI

(resonance-enhanced multiphoton ionization) [2] and

the recently developed CRDS (cavity ringdown

spectroscopy) [3], the most versatile and reliable

technique for concurrent determination of a total rate

constant and product branching ratios must be timeresolved

mass spectrometry. There are two approaches

to achieving time-resolution which provides kinetic

information: the classical discharge flow method

employing a movable injector for molecular reactant

[4] (which is most useful for atom-molecular reactions)

and the pulsed laser photolysis for atom or free radical

generation [5]. Both techniques utilize the principles of

gas dynamics for product sampling in the form of

molecular beams so as to achieve sensitivity and

authenticity in the composition of products detected.

David Gutman and coworkers [5] first employed

the pulsed laser photolysis/mass spectrometry

(PLP/MS) for free radical kinetic measurements using

vacuum UV photoionization for radical detection.

They adopted the high-pressure supersonic sampling

method developed by Saalfeld and coworkers [6,7] at

the U. S. Naval Research Laboratory. Using a micronsize

conical sampling orifice prepared by laser drilling,

Saalfeld and coworkers were able to study lowtemperature

hydrocarbon combustion reactions under

atmospheric-pressure conditions.

965


In the present series of studies, we have employed

the PLP/MS technique for measurements of the total

rate constants and product branching ratios of the NH 2

+ NO x (x=1,2) reactions which are pivotal not only to

the efficiency of NH 3 as a deNO x agent, but also to the

chain-propagation reactions in AN/AP/ADN

(ammonium nitrate / ammonium perchlorate /

ammonium dinitramide) propellant systems. The total

rate constants and especially product branching ratios

for reactions (1) and (2):

Gas Inlet

Excimer

Laser

Pressure

Gauge

Dye Laser

(Optional)

QMS

Diffusion Pump

Movable

Thermocouple

Mechanical

Pump

Turbo Pump

NH 2 + NO

NH 2 + NO 2

k 1a

⎯⎯

→ N2 H + OH (1a)

k 1b

⎯⎯

→ N2 + H 2 O (1b)

k 2a

⎯ ⎯ → H 2 NO + NO (2a)

k 2b

⎯ ⎯ → N 2 O + H 2 O (2b)

have been controversial and are subjects of much

discussion in recent years. Our results obtained by

PLP/MS to be reviewed later conclusively show that

these two reactions occur primarily by the two

individual product channels as indicated. Our

quantitative data for the total rate constants, k 1 and k 2 ,

and the branching ratios for (1a) and (2b), help settle

the long standing controversies regarding these

quantities.

We have also applied the PLP/MS technique to

measure kinetic data for the reactions of C 6 H 5 radicals

which are much needed for kinetic modeling of soot

formation in fossil fuel combustion. We will review

the effort made to acquire these pivotal data which

cannot otherwise be measured with reliability and

reproducibility.

We will conclude our review of the technique by

presenting unpublished data on the application of

REMPI/MS for the detection of CH 3 radicals for

measurement of the rate constants of their

recombination and bimolecular reaction with NO 2 . The

versatility of the Saalfeld reactor employed in

conjunction with pulsed laser photolysis and laser

ionization foretells the yet untapped potential of the

PLP/MS technique.

The authors take pride in dedicating this chapter of

review to the progenitor of the reactor and sampling

technique, Dr. Fred E. Saalfeld, currently Technical

Director of the Office of Naval Research.

II. EXPERIMENTAL SECTION

The high pressure mass-spectrometric sampling

technique with pulsed laser photolysis (PLP/MS) was

employed for rate constant and/or product branching

ratio measurements. The sampling technique of

Saalfeld and coworkers [6,7] has been extensively

Figure 1. Schematic diagram of the PLP/MS

experimental apparatus.

utilized by Gutman [5], Koshi, Matsui and their

collaborators [8] for kinetic measurements. A

schematic diagram of the pulsed laser photolysis/massspectrometirc

apparatus is shown in Fig. 1. Radical

reactants were generated photolytically with an excimer

laser (Lambda Physik EMG 102) in a quartz tubular

Saalfeld-type reaction tube which has an inner diameter

of 10 mm and a length of 150 mm with a conical

sampling hole of 120 µm diameter at the center of the

reactor. The reactor was mounted perpendicularly to

the detection axis of a quadrupole mass spectrometer

(QMS, Extrel model C50) which detects the positive

ion signals generated by electron impact (EP) or

resonance enchanced multiphton inonization (REMPI).

For elevated-temperature experiments, the reaction tube

was heated with nichrome ribbon 0.15 mm thick and 15

mm wide, insulated with ceramic wool. By adjusting

the current with a variac through the heater, the

reaction tube temperature could be varied from 300 to

1200 K. The temperature was measured using a

movable type K thermocouple, located near the center

of the reaction tube with an accuracy and uniformity of

2 K. The reaction gas temperature can be readily

calibrated (or confirmed) with a unimolecular system

such as 1,3,5-trioxane (C 3 H 6 O 3 → 3CH 2 O) whose

kinetics are well characterized.

The detection chamber housing the mass

spectrometer was separated from the supersonic

expansion chamber, which holds the reaction tube, by a

skimmer (1 mm orifice, Beam Dynamics model 1)

mounted at the center of a metal plate. The skimmer

was placed 3.0 mm from the sampling hole of the

reaction tube. The expansion chamber was pumped by

an Edwards Diffstak model 160/700 diffusion pump

with a pumping speed of 1300 l/s to a chamber base

pressure of 10 -7 Torr. The detection chamber was

evacuated by a Leybold turbomolecular pump with a

speed of 1000 l/s to a chamber base pressure of 10 -8

966


H 2 O at 835 K

Torr. The reaction tube was pumped by an Edward

rotary vacuum pump with an oil trap to prevent the

back-diffusion of oil vapor. During the experiment, the

pressures in the expansion and detection chambers

were kept at 5-10 × 10 -5 and 5-10 × 10 -6 Torr,

respectively.

All experiments were carried out under slow-flow

conditions with as long as 30 msec resident time, which

is considerably longer than most radical reaction times,

1-20 msec. Mixing of reactants and the helium buffer

gas was achieved in a stainless bellow tube prior to the

introduction into the reaction tube. The concentration

of each individual molecular reactant (R) was obtained

by the following formula: [R] = 1.60 × 10 -7 (%)

PF R /TF T mol/cm 3 , where (%) is the percentage of R in

its gas mixture, P is the total reaction pressure in torr, T

is the reaction temperature, F R is the flow rate of each

gas mixture, and F T is the total flowrate of all gases.

The flowrates were measured by using mass

flowmeters (Brooks, Model 5850C and MKS, 0258C)

and the gas pressure was measured with an MKS

Baratron manometer.

III. KINETIC APPLICATIONS

III-1. NH 2 + NO x

The reaction of NH 2 with NO x has been considered

to be a key step in the thermal reduction of NO x by

NH 3 [9-12] and in the combustion of ammonium nitrate

(AN) and ammonium dinitramide (ADN) [13-15]

because of its high efficiency in producing atomic and

radical chain carriers, H and OH. The possible major

product channels are:

NH 2 + NO

NH 2 + NO 2

k 1a

⎯⎯

→ N2 H + OH (1a)

k 1b

⎯⎯

→ N2 + H 2 O (1b)

k 2a

⎯ ⎯ → H 2 NO + NO (2a)

k 2b

⎯ ⎯ → N 2 O + H 2 O (2b)

For these reactions, as alluded to above, there have

been controversies [16-41] regarding not only the total

rate constants, but also the branching ratios for the

formation of the radical products which affect strongly

the efficiency of NH 3 as a deNO x agent.

In our experiment the NH 2 radical was produced by

the photolysis of NH 3 at 193 nm. The conversion of

NH 3 by photodissociation was 1-8 % depending on the

reaction temperature and photolysis laser energy (~30-

40 mJ). The time-resolved concentration profiles of the

reactants and the products were directly measured in

order to determine the total rate constants for these

reactions in the temperature range of 300 - 1200 K

Signals(arbitrary units)

N 2 O at 570 K

-10 0 10 20 30 40

time (msec)

using various mixtures of NH 3 /NO x /He (mainly He

diluent).

NH 3

-1 0 1 2 3 4

time (msec)

NO at 340 K

NO 2 at 732 K

Figure 2. Typical time-resolved reactant and

product signals in the reactions of NH 2 + NO and NH 2

+ NO 2 .

ln (k 1

)

ln (k 2

)

30.0

29.5

29.0

28.5

28.0

30.0

29.5

29.0

28.5

(a) H G

F

E

B

(b)

A

D

28.0

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

1000/T (K)

Figure 3. Arrhenius plots for the total rate

constants of the reactions of NH 2 with NO (a) and NO 2

(b). (a) A, Ref. 32; B, Ref. 23; C, Ref. 16; D, Ref. 18;

E, Ref. 19; F, Ref. 20; G, Ref. 17; H, Ref. 21; ◦, this

work; —, fitted value. (b) ◦ and solid line, this work;

dashed line, Ref. 32; ×, Ref. 35; o, Ref. 37; ◊, Ref. 36;

∆, Ref. 33.

C

967


Figure 2 shows the typical time-resolved transient

signals of H 2 O and NO in the NH 2 + NO reaction, and

N 2 O and NO 2 in the NH 2 + NO 2 reaction obtained at

the selected temperatures. These positive ion signals

were obtained by electron impact ionization at 70 eV

followed by QMS mass selection. Transient signals

were typically averaged over 200-500 laser shots with a

repetition rate of 2 Hz and recorded on a Nicolet 450

Digital Waveform Acquisition System. As shown in

the inset of the figure, the NH 3 signal dropped

immediately after laser firing, remained flat for a

period of ~30 ms until a fresh sample filled the front

half of the reaction tube and returned to its initial level.

During the 30 ms-time period, kinetic and branching

ratio measurements were carried out. The repetition

rate of 2 Hz allowed enough time between pulses for

the NH 3 signal level to return to its initial value.

For the total rate constant measurements, the timeresolved

concentration profiles were fitted to the

kinetically modeled values with the SENKIN program

[42] using a set of reactions to simulate the kinetics of

the NH 3 /NO x /He system at each experimental

temperature and condition [43-46]. The solid curves

presented in Fig. 2 represent the modeled results. The

results of our total rate constant measurements are

summarized in Fig. 3 and a weighted least-squares fit

of the total rate constants (in units of cm 3 /mol-s)

obtained to the nonlinear Arrhenius equations yielded

k 1 = 8.29 × 10 13 T -0.53 exp(300/T)

k 2 = 8.10 × 10 16 T -1.44 exp(-135/T)

Product channel branching ratios in the NH 2 + NO

reaction were measured by the mass selected detection

of H 2 O, CO 2 and NO in the temperature range of 300-

1200 K. The CO 2 product in the NH 2 + NO reaction,

which resulted from the rapid reaction of OH with

added CO, is a convenient and reliable measure of the

OH radical present in the system [47]. In order to

kinetically model the values of α 1 (k 1a /k 1 ) and β 1

(k 1b /k 1 ), we kept the total rate constant (k 1 = k 1a+k 1b ) of

the NH 2 + NO reaction unchanged for each temperature

and only the relative values of k 1a and k 1b were varied.

The absolute number densities of each product were

calculated by using its signal amplitude at the plateau

of the product concentration profile and a carefully

prepared calibration gas mixture of H 2 O/CO 2 /He. In

order to account for the radical recombination and other

secondary reactions in the calculation of product or

reactant number densities, kinetic modeling was carried

out with the SENKIN program for each experimental

run. The kinetically modeled averaged values of α 1 are

presented in Fig. 4(a) for comparison with the results

reported by several investigators.

On the basis of our total rate constant for NH 2 +

NO and the values of α 1 determined in this study at

300-1200K and those recently reported by Glarborg at

1250-1370 K, the absolute rate constants for reaction

(1) were evaluated by least-squares analyses and were

given below (in units of cm 3 /mol-s) for kinetic

modeling:

k 1a = 7.55 × 10 7 T 1.22 exp(778/T)

k 1b = 9.70 × 10 15 T -1.23 exp(28/T).

α 1

β 2

0 400 800 1200 1600 2000 2400

1.0

(a)

0.8

0.6

0.4

0.2

0.0

0.8

0.6

0.4

0.2

(b)

0.0

0 200 400 600 800 1000 1200

Temperature (K)

Figure 4. Summary of product branching ratios for

NH 2 + NO → N 2 H + OH (α 1 ) and NH 2 + NO 2 → N 2 O

+ H 2 O (β 2 ) as functions of temperature. (a) dotted line,

Ref. 24; dashed line, Ref. 22; ∆, Ref. 21; ∇, Ref. 25;

◊, Ref. 10; +, Ref. 26; ◦, this work. (b) ◦, this work;

●, Ref. 38; ∆, Ref. 39; ∇, Ref. 40; dashed line, ref. 41;

solid line, this work.

Product channel branching ratios for the NH 2 +

NO 2 reaction were determined by the plateau or nearplateau

values of N 2 O signals using various mixtures of

NH 3 /NO 2 /He in the same manner of the NH 2 + NO

reaction. The resulting branching ratios are presented

in Fig. 4(b) and compared with previously reported

values. As shown in the figure, the branching ratio of

the N 2 O + H 2 O product channel, β 2 (k 2b /k 2 ), determined

968


y fitting the absolute number density of N 2 O to model

yield, was found to be 0.19±0.02 without significant

temperature dependence. Similarly, the alsolute rate

constants for the two channels of reaction (2) can be

given by the following equations in units of cm 3 /mol-s:

k 2a = 6.56 × 10 16 T -1.44 exp(-135/T)

k 2b = 1.54 × 10 16 T -1.44 exp(-135/T).

III-2. C 6 H 5 Radical Recombination Reaction

Phenyl radical is one of the most important reactive

species in the combustion of hydrocarbons, particularly

in relation to the formation of polycyclic aromatic

hydrocarbons (PAH’s) and the combustion of lead-free

gasoline in which small aromatics are used as additives

[48-52]. In these combustion systems, the phenyl

radical recombination reaction is one of the most

important steps because of its effects on phenyl radical

concentrations. However, the rate constant for the

recombination reaction had not been measured and was

assumed chronologically from 1 × 10 14 cm 3 /mol-s [53]

to 1 × 10 13 cm 3 /mol-s [54] and to 3 × 10 12 cm 3 /mol-s in

more recent high-temperature kinetic data analyses by

Stein and coworkers [55].

By PLP/MS, the rate constant for the recombination

of C 6 H 5 radicals was determined by measuring the

absolute yields of biphenyl in the presence of varying

amounts of NO, which competes with the

recombination process:

3

C 6 H 5 + C 6 H 5

⎯ → C 12 H 10 (3)

4

C 6 H 5 + NO

⎯ → C 6 H 5 NO (4)

The C 6 H 5 radical was generated by the photolysis of

C 6 H 5 NO (nitrosobenzene) at 248 nm [56-65]; the

conversion of C 6 H 5 NO at this wavelength with an

unfocused and 30-50 mJ KrF laser beam was in the

range of 20-40% with no evidence of secondary

photofragmentation of C 6 H 5 . The initial concentration

of C 6 H 5 in each experimental run was determined

reproducibly from the depletion of C 6 H 5 NO in the

presence of an excess amount of HBr diluted in He.

The known fast abstraction reaction [59], C 6 H 5 + HBr

→ C 6 H 6 + Br, prevents the facile recombination

reaction, C 6 H 5 + NO → C 6 H 5 NO [58], and other C 6 H 5

radical reactions from occurring so as to give correct

calculation of the initial concentration of C 6 H 5 ,

[C 6 H 5 ] 0 . For C 12 H 10 , the saturated vapor pressure at

room temperature (297 K) diluted with 50 Torr of He

was used as the calibration sample.

The typical time-resolved transient signals are

shown in Fig. 5. The rise of the C 12 H 10 signal is

Signals (arbitrary units)

attributable to the formation of C 12 H 10 from the

recombination reaction of C 6 H 5 and the decay of

C 6 H 5 NO signal indicates the depletion of C 6 H 5 NO by

photolysis. The results indicate that the initial

concentration of C 6 H 5 , [C 6 H 5 ] 0 , determined by the

depletion of C 6 H 5 NO in the presence of an excess

amount of HBr ([HBr]/[C 6 H 5 ] 0 > 300), however, is

always greater than the quantity: 2[C 12 H 10 ] t +

([C 6 H 5 NO] t - [C 6 H 5 NO] 0 ), where [C 12 H 10 ] t and

[C 6 H 5 NO] t are the concentrations of biphenyl and

nitrosobenzene measured in the plateau region of the

concentration time profiles, typically t ≈ 2 msec.

[C 6 H 5 NO] 0 is the concentration of nitrosobenzene

measured after photolysis in the presence of excess

HBr. Accordingly, [C 6 H 5 NO] 0 + [C 6 H 5 ] 0 represents the

concentration of nitrosobenzene before photolysis.

The apparent loss of the C 6 H 5 radical at time t is

attributable to the association reaction:

C 6 H 5 + C 6 H 5 NO

∆[C 6 H 5 ]

[C 6 H 5 NO] loss

[C 6 H 5 ] o

[C 6 H 5 NO] gain

[C 6 H 5 NO] t

[C 6 H 5 NO] o

-10 -5 0 5 10 15 20 25 30

time (msec)

2[C 12 H 10 ] t

[C 6 H 5 NO] t - [C 6 H 5 NO] o

Figure 5. Typical time-resolved massspectrometric

transient signals of C 12 H 10 and C 6 H 5 NO.

5

⎯ → (C 6 H 5 ) 2 NO (5)

producing the biphenyl nitroxide radical, which had

been previously detected in solution at room

temperature [66]. Our search for m/z = 184 indeed

revealed the presence of the species, which was not

present before or after photolysis in the presence of the

excess amount of Hbr.

In order to account for the mass balance of the

C 6 H 5 , we write:

∆[C 6 H 5 ] = [C 6 H 5 ] 0 - 2[C 12 H 10 ] t

- ([C 6 H 5 NO] t - [C 6 H 5 NO] 0 ) (6)

969


32

where ∆[C 6 H 5 ] is the disappearance of C 6 H 5 through

reaction (5) which consumes both C 6 H 5 and C 6 H 5 NO

(see Fig. 5). Again, by mass balance,

[C 6 H 5 ] 0 = 2[C 12 H 10 ] t + [C 6 H 5 NO] gain + [(C 6 H 5 ) 2 NO] t

(7)

and

ln (k 3 )

31

30

29

[C 6 H 5 NO] t - [C 6 H 5 NO] 0

= [C 6 H 5 NO] gain - [C 6 H 5 NO] loss

= [C 6 H 5 NO] gain - [(C 6 H 5 ) 2 NO] t (8)

where [C 6 H 5 NO] gain is the formation of C 6 H 5 NO by

reaction (4) and [C 6 H 5 NO] loss is the consumption of

C 6 H 5 NO by reaction (5). Combining eqs. (7) and (8)

with eq. (6) and noting that [C 6 H 5 NO] loss is the same as

[(C 6 H 5 ) 2 NO] t , we obtain

28

0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6

1000/T (K)

Figure 6. Arrhenius plots for the rate constants of

C 6 H 5 + C 6 H 5 (◦) and C 6 H 5 + C 6 H 5 NO (●) reactions.

Dash-dotted line, Ref. 68 for the C 6 H 5 + C 6 H 5 reaction,

dashed line, Ref. 79 for the C 6 H 5 + C 6 H 5 reaction;

solid and dotted lines, this work.

∆[C 6 H 5 ] = 2[(C 6 H 5 ) 2 NO] t (9)

The mass balance for C 6 H 5 is then

or

[C 6 H 5 ] 0 = 2[C 12 H 10 ] t + ([C 6 H 5 NO] t - [C 6 H 5 NO] 0 )

+ 2 [(C 6 H 5 ) 2 NO] t

[(C 6 H 5 ) 2 NO] t = 1/2 {[C 6 H 5 ] 0 - 2[C 12 H 10 ] t

- ([C 6 H 5 NO] t - [C 6 H 5 NO] 0 )} (10)

We kinetically modeled the yields of C 12 H 10 ,

C 6 H 5 NO, and (C 6 H 5 ) 2 NO, which was obtained by Eq.

(10) using experimentally measured yields of [C 6 H 5 ] 0 ,

[C 12 H 10 ] t and [(C 6 H 5 ) 2 NO] t , at time t with the following

three-step mechanism:

3

C 6 H 5 + C 6 H 5

⎯ → C 12 H 10 (3)

4

C 6 H 5 + NO

⎯ → C 6 H 5 NO (4)

5

C 6 H 5 + C 6 H 5 NO

⎯ → (C 6 H 5 ) 2 NO (5)

by adjusting the values of k 3 and k 5 with the known rate

constant for reaction (4), k 4 = 2.7 × 10 12 exp(433/T)

cm 3 /mol-s [58]. The averaged values of k 3 and k 5

obtained from the modeling of kinetic data covering

five temperatures between 300 and 500 K are

graphically presented in Fig. 6. A weighted leastsquares

analysis of the data by convoluting the reported

error for k 4 [58] gave for reaction (3):

k 3 = (1.39±0.11) × 10 13 exp[-(56±33)/T] cm 3 /mol-s

and for reaction (5):

k 5 = (4.90±0.19) × 10 12 exp[(34±16)/T] cm 3 /mol-s.

III-3. C 6 H 5 + H 2 and CH 4 Reactions

For the kinetic studies of phenyl radical reactions,

we have carried out experiments using the cavity

ringdown spectrometry (CRDS) technique, covering a

typical temperature range of 298-523 K [56-65].

However, temperature broadening of the rovibronic

transition of C 6 H 5 in the visible region limits its kinetic

measurements by CRDS to about 523 K, above which

its S/N ratio deteriorates rapidly with temperature.

This temperature limit (T ≤ 523 K) precludes the

studying of slower reactions such as C 6 H 5 + H 2 and

CH 4 . In order to circumvent the shortcoming, we

employed the PLP/MS technique for kinetic

measurement of these reactions. The method

effectively extends the kinetically useful temperature

range to ~1000 K.

The pulsed photolysis of C 6 H 5 COCH 3 at 193 nm

was employed as the C 6 H 5 radical source. The mole

fraction of C 6 H 5 COCH 3 was typically < 0.5 % and that

of H 2 or CH 4 was > 75 % with [H 2 or

CH 4 ]/[C 6 H 5 COCH 3 ] > 150. The conversion of

C 6 H 5 COCH 3 by the unfocused ArF laser beam ranged

from 15 % to 40 %. The mechanism for the

fragmentation of C 6 H 5 COCH 3 at 193 nm has been

studied by using NO or HBr as the C 6 H 5 radical

scavengers. The kinetic modeling of measured yields

of C 6 H 5 NO or C 6 H 6 under fully inhibited conditions

970


evealed that 60 - 80 % of the fragmentation reaction

gave rise to C 6 H 5 . This result is consistent with the

measured yield of C 6 H 5 CH 3 without NO or HBr. The

introduction of an excess amount of NO, for example,

eliminated the formation of toluene [67].

In the photo-initiated reaction of C 6 H 5 COCH 3 in

the presence of excess amounts of H 2 or CH 4 , the

production of C 6 H 6 was found to be influenced by the

following primary processes:

C 6 H 5 + C 6 H 5 → C 12 H 10 (3)

C 6 H 5 + H 2 → C 6 H 6 + H (11)

C 6 H 5 + CH 4 → C 6 H 6 + CH 3 (12)

C 6 H 5 + CH 3 → C 6 H 5 CH 3 (13)

CH 3 + CH 3 → C 2 H 6 (14)

In the absence of H 2 or CH 4 , the major molecular

products from the C 6 H 5 COCH 3 photolysis were

C 6 H 5 CH 3 , C 2 H 6 and C 12 H 10 (which was not

quantitatively determined in this study), with a trace

amount of C 6 H 6 . Addition of H 2 /CH 4 to the system

noticeably reduced the yields of C 12 H 10 and C 6 H 5 CH 3

with a concomitant increase in the yield of C 6 H 6 . The

measurement of toluene in the present experiment is

very useful because its formation, solely by the

recombination of C 6 H 5 with CH 3 , allows us to reliably

monitor, albeit indirectly, the concentration of the C 6 H 5

radical. Since both radicals are expected to be formed

with equal concentrations initially and the rate constant

for the CH 3 radical recombination reaction is wellestablished,

the concurrent modeling of the yields of

C 6 H 6 and C 6 H 5 CH 3 provides reliable rate constants for

reactions (11) and (12).

The kinetically modeled values of k 11 based on the

absolute yields of C 6 H 6 and C 6 H 5 CH 3 are summarized

in Fig. 7(a) for comparison with those obtained by

pyrolysis/FTIR spectrometry (P/FTIR) [67] in our

laboratory and by Troe and coworkers using UV

absorption spectrometry carried out in a shock tube at

temperatures between 1050 and 1450 K [68]. The three

independent, more direct measurements agree closely

with our theoretically predicted values covering the

entire temperature range investigated, 548-1450 K, k 11

= 5.72 × 10 4 T 2.43 exp(-3159/T) cm 3 /mol-s [69]. In the

figure, we also compare these three sets of

experimental data and the theoretically predicted curve

with the existing, mostly kinetically modeled results

obtained by shock-heating of C 6 H 6 at high temperatures

[70-72]. Among them, the data of schlieren

measurements by Kiefer and coworkers [72] agree

most closely with the theory.

The rate constants for the C 6 H 5 + CH 4 reaction at

various temperatures obtained by PLP/MS are also

presented in Fig. 7(b) together with P/FTIRS and other

ln(k 11

)

ln(k 12

)

32

30

28

26

24

22

20

18

16

28

26

24

22

20

18

16

3

1

2

14

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

1000/T (K)

Figure 7. Arrhenius plots for the rate constants of

C 6 H 5 + H 2 (a) and C 6 H 5 + CH 4 (b) reactions. (a) ¨,

this work; ◦, Ref. 67; ∆, Ref. 68; ∇, ref. 53 using the

rate constant for the recombination of C 6 H 5 reported by

Park and Lin (ref. 74); 1, theoretical results of Mebel et

al., ref. 69; 2, ref. 71; 3, ref. 72. (b) ¨, this work; ◦,

Ref. 73; ∆, Ref. 68; ∇, ref. 53 using the rate constant

for the recombination of C 6 H 5 reported by Park and

Lin (ref. 74); solid curve, theoretical results, Ref. 73.

existing data [53,68] for comparison with the

theoretically predicted result, k 12 = 1.42 × 10 6 T 2.02

exp(-5090/T) cm 3 /mol-s [73]. In Fig. 7(b) we have

included two existing sets of kinetic data reported by

Duncan and Trotman-Dickenson [53] employing

steady-state UV photolysis of C 6 H 5 COCH 3 in the

presence of CH 4 and by Troe and coworkers [68] using

the shock tube/UV absorption spectroscopy carried out

in the temperature range 1050 - 1450 K. The result of

Duncan and Trotman-Dickenson [53] was evaluated

with reference to the C 6 H 5 recombination reaction,

whose rate constant was assumed to be k 3 = 1×10 14

cm 3 /mol-s, independent of temperature. We rescaled

their CH 4 abstraction rate constant by using our

reported C 6 H 5 recombination rate constant, k 3 =

1.39×10 13 exp(-55/T) cm 3 /mol-s [74], as mentioned

above.

971


III-4. CH 3 + CH 3 and CH 3 + NO 2 Reactions

The CH 3 + NO 2 reaction is an important step in

NO x reburning in hydrocarbon combustion process.

This involves recycling previously formed NO x into a

hydrocarbon-rich combustion zone. The reaction of

CH 3 with NO 2 involves the following competitive

processes:

CH 3 REMPI Signals

333.0 333.2 333.4 333.6 333.8 334.0

wavelength (nm)

CH 3 + CH 3 → C 2 H 6 (14)

CH 3 + NO 2 → CH 3 O + NO (15a)

→ HNO + CH 2 O (15b)

→ CH 3 NO 2

(15c)

In this study the methyl radical was produced from

acetone photolysis at 193 nm. The detection of acetone

was accomplished by electron impact ionization at 60

eV and the detection of the CH 3 radical and its decay

was accomplished by (2+1) REMPI/MS [75,76] using

the 333.4 nm line of p-terphenyl dye in a Lambda

Physik FL3002 dye laser pumped by an EMG201

excimer laser. In the inset of Fig. 8 we present a

wavelength scan of the CH 3 radical covering the 333-

334 nm region. The major peak appearing at 333.4 nm

corresponds to the 0 0 0 band of the 3p 2 A 2 " ← X 2 A 2 "

transition. The CH 3 ion signal was measured as a

function of time between the photolysis and REMPI

lasers. The ion signal from the channeltron was

amplified by an SRS445 fast gated amplifier and

recorded as SRS440 photon counter. Step sizes of 400

ns were used with 200 laser pulses per step.

The decay of the CH 3 radical produced from the

photodissociation of 9.50 × 10 -12 mol/cm 3 of acetone

diluted with 11 Torr of He in the absence of NO 2 is

shown in Fig. 8. The nonlinear least-squares fitting of

the observed decay curve to the integrated second-order

rate equation,

[CH 3 ] t = [CH 3 ] 0 / (1 + 2k 14 t [CH 3 ] 0 ) (16)

gives k 14 = 2.89 × 10 13 cm 3 /mol-s at 297 K and P = 11

Torr (He). In the above equation, [CH 3 ] 0 and [CH 3 ] t

are the concentration of the CH 3 radical at t = 0 and t =

t, respectively. The value of the CH 3 recombination

rate constant given above agrees excellently with those

reported by Gutman, Pilling and coworkers [77], 2.29 ×

10 13 and 2.71 × 10 13 cm 3 /mol-s at 296 K, in the

presence of 16.8 Torr He and Ar, respectively.

The rate of the CH 3 + NO 2 reaction is known to be

fast. At low pressures, the reaction is dominated by the

association/decomposition processes,

CH 3 + NO 2 → CH 3 ONO † → CH 3 O + NO (15a)

→ HNO + CH 2 O (15b)

0 10 20 30 40

Delay (msec)

Figure 8. The decay of CH 3 REMPI signals at

333.4 nm following the photodissociation of 9.50×10 -12

mol/cm 3 acetone. ◦, [NO 2 ]=0; o, [NO 2 ]=2.00×10 -11

mol/cm 3 . Inset: CH 3 REMPI spectrum from 333.0 to

334.0 nm.

and the association process,

† +

CH 3 + NO 2 → CH 3 NO 2 ⎯ ⎯ M → CH 3 NO 2 , (15c)

becomes pressure-dependent and less important. The

total rate of the CH 3 + NO 2 reaction can be readily

measured by using varying excess amounts of NO 2 .

Figure 8 shows the typical CH 3 decay with and without

NO 2 added. Least-squares analyses of the CH 3 radical

decay give rise to k 15 = 1.52 × 10 13 , 1.44 × 10 13 , 1.39 ×

10 13 and 1.24 × 10 13 cm 3 /mol-s at 301, 417, 512 and

620 K, respectively, as shown in Fig. 9. The slight

k 15 (cm 3 /mol-s)

14.0

13.5

13.0

12.5

12.0

11.5

11.0

k 15a

k 15b

10.5

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

1000/T (K)

Figure 9. Arrhenius plot of the rate constant for the

CH 3 + NO 2 reaction. ∆, Ref. 80; •, Ref. 78; ◦, this

work; solid curves, RRKM results (Ref. 78); dashed

curve, Ref. 81,

972


decline of this rate constant with temperature in this

temperature regime is consistent with the theoretical

prediction based on the RRKM theory [78].

IV. CONCLUDING REMARKS

In this review, we have briefly summarized our

recent kinetic data obtained with the pulsed laser

photolysis/mass spectrometric technique. We have

illustrated that very high-quality kinetic data for such

controversial reactions as NH 2 + NO x (x=1,2), which

are pivotal to the combustion of energetic materials

(AN, AP and ADN) as well as to the thermal reduction

of NO x by NH 3 , can be reliably obtained with the

technique through the judicious choice of experimental

conditions and careful concentration calibrations for the

species measured.

By the conventional electron impact ionization for

the stable products formed in the studies, we can not

only determine the total rate constants for the NH 2 +

NO x reactions to the accuracy equal or better than those

acquired by laser-induced fluorescence or laser

resonance absorption, but also determine their product

branching ratios to an unparalleled level of reliability.

For the crucial reactions involving phenyl radicals,

the recombination and the reactions with hydrogen and

methane (which have not been reliably determined by

existing techniques for comparison with the results of

quantum and statistical-theory calculations), we are

able to acquire by PLP/MS for the first time reliable

Arrhenius parameters over a wide range of temperature.

Agreement between theory and experiment for these

reactions has been excellent.

We have also demonstrated that high-quality

kinetic data for radical species, such as CH 3 which can

be sensitively detected by REMPI/MS, can also be

measured by the PLP(pump)-REMPI/MS(probe)

method. Here we replace the conventional VUV

atomic resonant lamp employed by Gutman and

coworkers [5,77] with a high-power tunable dye laser

for species ionization.

A natural extension of the REMPI and/or the VUV

resonance lamp photoionization technique is the

employment of a tunable VUV laser for the ionization

process. The use of VUV laser for one-photon

photoionization should improve significantly the

efficiency of ionization (over that of REMPI) and

reduce the extent of ion-fragmentation which is a

recurring problem with the electron impact ionization

method. With a tunable VUV laser, one may detect

multiple reactants and products at once with a medium

resolution time-of-flight mass spectrometer. For

example, in the reaction of C 6 H 5 with C 2 H 2 which is

relevant to incipient soot formation, all reactive species

but H atoms in the system:

C 6 H 5 + C 2 H 2 → C 6 H 5 C 2 H 2 * → C 6 H 5 C 2 H + H

+

⎯ ⎯ M → C 6 H 5 C 2 H 2

can be detected, in principle, by tunable VUV laser

ionization. Such a measurement can provide crucial

data for total radical loss rates and the formation rates

of radical and molecular products as well.

Acknowledgments

The authors gratefully acknowledge the support of

the NH 2 -NO x study from the Office of Naval Research

under the direction of Drs. R. S. Miller and J.

Goldwasser and the sponsorship of the aromatic radical

kinetic studies from the Basic Energy Sciences,

Department of Energy, under the direction of Dr. W.

H. Kirckhoff. This article is dedicated to Dr. Fred E.

Saalfeld, Technical Director of the Office of Naval

Research, who developed the high-presure reactor/

supersonic sampling mass spectrometric technique.

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