Spectra of Rydberg States of Collision Dimers Xe2 in the VUV Range

EAWCD15-yylee
Spectra of Rydberg States of Collision Dimers Xe 2 in the VUV Range
Yin-Yu Lee
National Synchrotron Radiation Research Center, Hsinchu Science Park, Hsinchu
30076, Taiwan
Abstract- The transition of Xe 2 Rydberg states to the autoionization Rydberg states
are studied by measuring xenon ions from IR-selective dissociative ionization of high
energy xenon collision pairs.
Rydberg xenon dimers are generated in the collisions
of ground state atoms with high Rydberg xenon atoms excited by synchrotron
radiation in the effusive jet.
Subsequently, high-lying dimers are excited to
autoionization states overlapping with xenon dimer autionizing continuum or ionic
dimer states and dissociably ionized to Xe and Xe + . With polarization measurement
of ionic xenon, autoionization Rydberg states are assigned to be the dissociation limit
of Xe* nd’ and nf’ states.
1

Rydberg states are the excited states next to the valance orbitals in the atoms or
molecules, and energy can be described in Rydberg equation:[1]
E = T – R(n-) -2
Neutral atomic or molecular Rydberg states overlapping with ionic continuum are
Autoionization Rydberg states (ARS) which have importance in understanding the
interactions between the Rydberg electron and ionic core. The ARS of diatomic
molecules which usually have more rigid structure than polyatomic ones can be
measured by photoionization mass spectroscopy (PIMS), photoelectron spectroscopy
(PES), absorption spectroscopy and some optical techniques.[2] Special kind of
diatomic molecules are rare gas dimers forming with atom-atom van der Waal’s
attraction which is non chemical-bonded and have unstable ground states and more
stable long-lived excited states. Rare gas dimers are fully occupied with electrons in
valance shell, and their autoionization Rydberg states are respected to be approxmate
to atomic Rydberg series of rare gases with typical Fano line shapes which are
induced in the interactions of discrete and continuum states above the ionic energy of
the Xe + 2 A 2 Σ + 1/2u state.[3-5] Xenon is a much heavier atom in the rare gas group and
easily forms dimer due to the Van der Waal’s mass factor under the condition of room
temperature. All the Rydberg states of Xe 2 are located in the VUV energy region
and some of them are more stable than ground state.
Consequently, the xenon
dimers are powerful materials for generating the VUV light source of laser and lamp
now.[6]
Previous measurements of Xe 2 autoionizing states were studied by one photon
excitation to vicinity of ns’ and nd’ xenon atomic states between the Xe + ( 2 P 3/2 ) and
Xe + ( 2 P 1/2 ) levels.[7-9]
Some gerade states overlying with ionic layer are
subsequently studied by two-photon excitation.[10-12] But direct survey of dimers
Rydberg and autoionizing Rydberg states is still problematical attributed to poor
2

Frank-Condon overlap from ground state, competition of dissociative reactions, and
optical limitation in VUV energy region.
Keto’s group presented that the collision dimers would be formed in energy decay
processes of excited xenon atoms colliding with ground state ones and Rydberg states
of Xe 2 can be studied.[13] To solve the problems, we establish an experimental
setup which has effusive jet and photoionization mass spectrometer with synchrotron
radiation used as the exciting light source.
Xenon atoms are excited by synchrotron
radiation and then collide with ground state atoms forming excited dimers in effusive
beam.
High Rydberg states Xe 2 * which are excited to autoionization series by IR
laser following with dissociative ionization to have Xe + . Consequently, autoionizing
states of dimers could be deduced by measuring the xenon ions.
The vacuum chamber equipped with an effusive nozzle and photoionization mass
spectrometer. Pure xenon gases with back pressure 2-10 torr expanded into vacuum
chamber through effusive beam intersected with synchrotron radiation and infrared
light at ion region in mass spectrometer.
Synchrotron radiation light in vacuum-UV
energy region was separated by a monochromator equipped with a cylindrical golden
grating in resolving power E/ΔE = 20000 at 10 eV with the entrance and exit slit of 50
and 100 μm. Xenon atoms are selectively excited to ns, nd, ns’ and nd’ atomic
Rydberg states in the wave number range of 85000-97000 cm -1 , respectively.[14]
The excited atoms collide with ground state xenons constructing the high Rydberg
xenon dimer states on the top of effusive jet. Near infrared light generated from a
continuous wave Ti-sapphire laser (Coherent 899-21 and Spectra-Physics, Tsunami)
pumped by an 532 nm Nd:YAG laser (Spectra-Physics, Millenia) is used as ionizing
dissociation source of xenon collision pairs.
The output wavelength is tuned by a
birefringent filter yielding the spectral resolution of 0.25 cm -1 in the scanning range of
11800-13700 cm -1 of Coherent 899-21 and resolution about 1.5 cm -1 in the scanning
3

ange of 13300-14200 cm -1 of Tsunami laser for the present work.
The absolute
position of laser wavelength is measured by a wavemeter (EXFO 4550) with accuracy
of 0.02 cm -1 . The laser output is linearly polarized and rotatable with a half-wave
Fresnal rhomb polarizer.
The IR laser beam counter-propagated with synchrotron
radiation beam was softly focused to a spot diameter 0.5 mm in the mass spectrometer
extraction region with electronic filed ~10V/cm.
Ions xenon produced from
ionization dissociation were detected with a quadrupole mass filter set mass = 131.
The selected ions were collided with a diode applied high voltage for conversion to
electrons in a channeltron. The output signal current was amplified and then counted
with a counter.
The ground states of xenon dimer is X 1 Σ + g (0 + g ) with an electronic configuration
(σ g 5s) 2 (σ u 5s) 2 (σ g 5p) 2 (π g 5p) 4 (π u 5p) 4 (σ u 5p) 2 , a shallow potential energy well and a large
intermolecular distance. The removal of one electron from each of the valance
molecular orbitals lead to six electronic states for Xe + 2 A 2 +
Σ 1/2u , B 2 Π 3/2g , B 2 Π 1/2g , C
2 Π 3/2u , C 2 Π 1/2u , and D 2 Σ + 1/2g ; these states are displayed with the Hund’s rule notation
[ 2S+1 Λ Ωg/u ] in which Ω denotes the total electronic angular momentum with Λ defined
as the total orbital angular momentum.
The symbol Ω g/u is suggested as a
well-defined quantum number than Λ and Σ (projection of electron spin S along the
internuclear axis).
The lowest four xenon dimmer ion electronic states correspond to
the Xe( 1 S 0 )+Xe + ( 2 P 3/2 ) limit whereas the remaining two dissociate to Xe( 1 S 0 )+
Xe + ( 2 P 1/2 ) state.
Xe 2 Rydbreg states converging to six electronic ion states are
appropriately treated as an ionic states and a Rydberg electron state.[5] Congestion
of autoionization Rydberg states above the first ionic state lying with ionic continuum
dissociate to Xe( 1 S 0 ) + Xe(nl[K] J , n≧7) or Xe(nl’[K] J , n≧7) limit.
The generation of high Rydberg xenon dimers with the excitation of xenon
atomic levels above 85000 cm -1 within the collision processes on the top of effusive
4

jet, the signal xenon ions are monitored in m/z =131 and arise from the 8s[3/2] 1 state
(90932.432 cm -1 ) with infrared dissociative ionization. The spectra of ion signal are
shown in FIG. 2 in which the horizontal scale is infrared light energy.
Despite the
transition of Rydberg to autoionization states are in the present work, it should be
noted that atomic even-parity autoionizaiton Rydberg states and dimer Rydberg states
to ionic states are possibly included superimposing on the xenon ionic continuum
level.
Two-photon resonant-enhanced excitation of xenon atoms are indicated by green
ticks whereas the prime means the series converging to Xe + ( 2 P 1/2 ) limit. Xenon
atoms are step-wise pumped by synchrotron radiation single-photon excitation to nd
and ns states where J is equal to 1, and subsequently excited to even-parity np’[3/2] 2,1 ,
np’[1/2] 1,0 and nf’[5/2] 2 states.[14] Here the intense atomic peaks are assigned np’
series because of good excitation cross section of s’p’ transition.
These series
above the IE of xenon spreading over the ionic state with specific description of Fano
formula are autoionizaiton Rydberg states and consequently could be measured by
mass spectrometer.
The observations of the peaks with FWHM ~5cm -1 in FIG. 4 via intermediate
atomic Xe*5p 5 12s[3/2] 1 state are marked in Arabic numerals and assigned to be the
Rydberg states in the energy region of 83700-86000 cm -1 . Their existences refer to
the relaxation of collision pairs to the relatively stable Rydberg states of xenon
dimers.[15] These Rydberg states are excited to the autoionizing levels coupling
with the Xe + 2 B 2 Π 1/2g state:
Xe ( 1 S 0 ) + hυ (SR,VUV) Xe**
Xe** + Xe → Xe 2 **
Xe 2 ** → Xe 2 * + hυ (em, IR)
Xe 2 * + hυ (laser, IR) → Xe + + Xe
5

or
Xe ( 1 S 0 ) + hυ (SR,VUV) → Xe**
Xe** → Xe* + hυ (em, IR)
Xe* + Xe → Xe 2 *
Xe 2 * + hυ (laser, IR) → Xe + + Xe
Both atomic and collision dimer emission can occur in the decay processes.
Nonradiative path are possibly included in the reaction.
These bands in which the
energy are determined in the difference from Xe + ( 2 P 3/2 ) at 97833.787 cm -1 and the
marked from high IR to low IR energy ( band energy from low to high). These
results could be consistent with previous studies of absorption, laser-induced
fluorescence spectra, and theoretical calculation.[16-19]
In FIG. 4 band 1-13 in the
energy region of 83700-85070 cm -1 are covered in the absorption spectra[16], and
subsequently band 2 is assigned to 0 u
+
state which dissociates to Xe( 1 S 0 ) +
Xe*5d[3/2] 1 .[18]
Band 13 is 0 u + or 1 u states which has good agreement with LIF
experimental results, and band 11 is compatible with theoretical calculation.[19]
Others are completely new observed Rydberg states which are not measured
previously due to poor Frank-Condon overlap from dimer ground states.
The intense bands with FWHM>10 cm -1 and pairing structure in FIG. 3 and 4 are
labeled by Roman numbers and listed in Table I. These bands appear above the
energy of atomic states 8s[3/2] 1 (90932.432 cm -1 ) states where the energy is slightly
higher than the ionic threshold of xenon dimers.[5] These bands are excluded from
xenon atomic lines because of no fitting energy difference from these atomic states
and unseriously polarization effect.
These band have Fano line profile and are
indentified as the transitions of Rydberg states to autoionzation Rydberg states. The
signals are from this proposed processes:
Xe + hυ (SR,VUV) → Xe**
6

Xe** + Xe → Xe 2 **
Xe 2 ** + hυ (IR) → Xe 2 ***(ARS)
→ Xe*(ARS)+Xe → Xe + + Xe
The most intense signals are observed via the atomic states 7s’[1/2]1 and 10d[1/2] 1 at
the energy of 95800.584 and 95912.880 cm -1 , respectively.
Band I and IV at the
position 13937 and 13476.4 cm -1 vary with weaker horizontal and vertical polarizing
effect, and band III with broad shape has no obvious polarization dependence.
In
contrast band VII at 12245.2 cm -1 with extensity quantity differs apparently with light
polarization. The bands V, VI, and VIII have and pairing structure in which the
separation of doubly pairs is about 30 cm -1 . The lower energy items reveal the more
unambiguously intense variety of laser polarization than the higher ones, so the
vibrational progressions are excluded in this case due to the polarization dependence
of laser light.
Based on the results of Lipson and his co-workers’ works, they show
that the spectra of Xe 2 autoionizing Rydberg states above the adiabatic IE near the
dissociation limit of atomic 4f states are with evident polarization effect. These
bands are assigned as autoionization Rydberg states complying with Lipson’s
exposition.[11,12]
Despite the initial collision dimer states are generated via various xenon Rydberg
states in which energy range from 90000 to 97000 cm -1 , their ionic signal from IR
excitation are analogous with similar FWHM. It is considered that the relaxation
processes occur on the top of the effusive beam and consequently collision pairs
quench to a lower and more stable state.
This assumption of the same initial state is
in agreement with the collisional energy transfer procedures of the xenon Rydberg
states in Museur’s work.[15] The band width represent the initial state overlap with
xenon ionic states A 2 Σ + 1/2u and have strong intensity via 7s’[1/2] 1 state due to the 2 P 1/2
ionic core interference. This indicates that these ARS converge to the Xe + 2 C 2 Π 1/2u
7

electronic states corresponding to the Xe + ( 2 P 1/2 ) + Xe( 1 S 0 ) limit.
The appearance of
the xenon dimer signals at the atomic states 8s[3/2] 1 in which the energy is 90932.432
cm -1 . This energy range of upper states is slightly lower than the 90932.432 cm -1
adding the IR energy and locate in the energy region of 103000-110000 cm -1 . The
upper dimer states have predominant dissociative ionization with IR selective
excitation and assumed to have weak interactions with the dimer ionic Σ and Π states.
This infers that the final autoionization states could be the Δ or Φ state which
indicates the dissociation limit related to xenon nd’ or nf’ levels consisting of δ and φ
components. These ARS are assigned near the dissociation limit Xe*5-6f’ ad 7-14d’
according to the energy population. The broad band width is attributed to the factors
of lifetime of upper states, Frank-Codon transition from Rydberg, and etc.
Xenon atoms are excited to one specific J states by high resolution synchrotron
radiation with horizontal direction.
In this excitation energy range, every atomic
states can be separated well. After colliding process, dimers Rydberg states kept the
original characteristics from Rydberg xenon atomic states.
The transitions from
these ARS with specific polarization could be induced or reduced to upper ARS by
polarized laser light.
We studied the Xe 2 Rydberg and autoiomization Rydberg states from IR
excitation of collision dimmers. The resolution in our spectrum is determined by the
IR laser which has narrow line width about 0.2 cm -1 and the detective problems are
resolved using MS. Synchrotron radiation gives good VUV light source which have
high resolution with a monochromator.
This setup successfully resolves optical
problems and measures the Rydberg states and ARS in this work.
8

References
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[2]Vacuum Ultraviolet Photoionization and Photodissociaiton of molecules and
clusters, edited by C. Y. Ng (World Scientific, 1991)
[3] U. Fano, Phys. Rev. 124, 1866 (1961)
[4] O. Zehnder et al, J. Chem. Phys. 128, 234306 (2008)
[5] R. S. Mulliken, J. Chem. Phys. 52, 5170 (1970)
[6] M. Salvermoser and D. E. Murnick, Appl. Phys. Lett. 83, 1932 (2003)
[7] C. Y. Ng et al., J. Chem. Phys. 65, 4327 (1976)
[8] P. M. Dehmer, and S. T. Pratt, J. Chem. Phys. 75, 5265 (1981)
[9] K. Norwood, G. Luo, and C. Y. Ng, J. Chem. Phys. 90, 4689 (1989)
[10] S. T. Pratt, P. M. Dehmer, and J. L. Dehmer, Chem. Phys. Lett. 165, 131 (1990)
[11] X. K. Hu, et al., J. Chem. Phys. 109, 3944 (1998)
[12] X. K. Hu et al., Chem. Phys. Lett. 353, 213 (2002)
[13] N. Böwering, T. D. Raymond, and J. W. Keto, Phys. Rev. Lett. 52, 1880 (1984)
[14] NIST Atomic Spectra Database (version 4.0), [Online]. Available:
http://physics.nist.gov/asd (2010)
[15] L. Museur et al., J. Chem. Phys. 101, 10 548 (1994)
[16] M. C. Castex, Chem. Phys. 5, 448 (1974)
[17]K. Tsukiyama and T. Kasuya, J. Mol. Spectrosc. 151, 312 (1992)
[18]D. M. Mao et al, J. Mol. Spec. 181, 435 (1997)
[19] C. Jonin and F. Spiegelmann, J. Chem. Phys. 117, 3059 (2002)
9

Table I.
The energy and FWHM of Xe 2 autoionization Rydberg states.
Figure Captions
FIG. 1 Energy diagram of xenon atomic Rydberg, dimer Rydberg, and ionic states.
Here the xenon atoms are excited to Rydberg states and then collide with neutral
atoms to form Rydberg dimers.
These high lying dimers are excited to
autoionizaiton Rydberg states or shallow ionic states and ionizatively dissociate to
Xe + and Xe.
FIG. 2 Spectra of IR selective Rydberg states forming via Xe*7s’[1/2] 1 to
autoionization Rydberg transition of xenon dimers by measuring Xe + signals from
ARS dissociative-ionization to Xe + + Xe in the IR energy region of 11800-13500 cm -1 .
The Fig. 2(a) show that the horizontal -polarized SR light excitation of atoms and
horizontal-polarized laser excitation of Xe 2 Rydberg states, and the (b) are
horizontal-polarized SR light and vertical -polarized laser layout.
FIG.3 Spectra of IR selective Rydberg states forming via Xe*7s’[1/2] 1 to
autoionization Rydberg transition of xenon dimers by measuring Xe + signals from
ARS dissociative-ionization to Xe + + Xe in the IR energy region of 13300-14200 cm -1 .
The Fig. 2(a) show that the horizontal -polarized SR light excitation of atoms and
horizontal-polarized laser excitation of Xe 2 Rydberg states, and the (b) are
horizontal-polarized SR light and vertical -polarized laser layout.
FIG. 4 Spectra of IR selective Xe 2 Rydberg states forming via Xe*12s[3/2] 1 to ARS
and ionic states transition by measuring Xe + signals from dissociative-ionization to
Xe + + Xe limit in the IR energy region of 11800-14200 cm -1 . The Xe 2 Rydberg
10

states signals are labled with blue bar, and their positions can be deduced by the
difference from E[Xe + ( 2 P 3/2 )] and IR energy.
The red marks are the peaks of
autoionization Rydberg states.
11

Table I.
The energy and FWHM of Xe 2 autoionization Rydberg states.
Band
IR Energy
(cm -1 )
Appearance Energy
(cm -1 ) & State
Total Energy
(cm -1 )
FWHM
(cm -1 )
Nearest
State
I 13937
5d’[3/2] 1
93618.24
107555 33 14d’
II 13837
III 13766
7s’[1/2] 1
95800.584
7d[1/2] 1
92128.287
109638 15
105894 85 9d’
IV 13476.4
8s[3/2] 1
90932.432
104408 14.3 8d’
V
13030.4 8s[3/2] 1 103920.2 15.6
12997.8 90932.432 103962.8 18.5
5f’
VI
12632,.5 8s[3/2] 1 103564.9 15.5
12661.4 90932.432 103593.8 18.0
7d’
VII 12245.2
5d’[3/2] 1
93618.24
105863.3 15.2 9d’
VIII
12098.2 7d[1/2] 1 104226.5 13.2
12127.8 92128.287 104256.1 16.6
6f’
12

FIG1. Energy diagram of xenon atomic Rydberg, dimer Rydberg, and ionic states.
Here the xenon atoms are excited to Rydberg states and then collide with neutral
atoms to form Rydberg dimers.
These high lying dimers are excited to
autoionizaiton Rydberg states or shallow ionic states and ionizatively dissociate to
Xe + and Xe.
13

FIG2.
Spectra of IR selective Rydberg states forming via Xe*7s’[1/2] 1 to
autoionization Rydberg transition of xenon dimers by measuring Xe + signals from
ARS dissociative-ionization to Xe + + Xe in the IR energy region of 11800-13500 cm -1 .
The Fig. 2(a) show that the horizontal -polarized SR light excitation of atoms and
horizontal-polarized laser excitation of Xe 2 Rydberg states, and the (b) are
horizontal-polarized SR light and vertical -polarized laser layout.
14

(a)
FIG. 3 Spectra of IR selective Rydberg states forming via Xe*7s’[1/2] 1 to
autoionization Rydberg transition of xenon dimers by measuring Xe + signals from
ARS dissociative-ionization to Xe + + Xe in the IR energy region of 13300-14200 cm -1 .
The Fig. 2(a) show that the horizontal -polarized SR light excitation of atoms and
horizontal-polarized laser excitation of Xe 2 Rydberg states, and the (b) are
horizontal-polarized SR light and vertical -polarized laser layout.
(b)
(a)
15

FIG. 4 Spectra of IR selective Xe 2 Rydberg states forming via Xe*12s[3/2] 1 to ARS or
ionic states transition by measuring Xe + signals from dissociative-ionization to Xe + +
Xe limit in the IR energy region of 11800-14200 cm -1 . The Xe 2 Rydberg states
signals are labled with blue bar, and their positions can be deduced by the difference
from E[Xe + ( 2 P 3/2 )] and IR energy. The red marks are the peaks of autoionization
Rydberg states.
16