VUV Spectroscopy of Atoms, Molecules and Surfaces

1.4 Comparison with other methods ... 13
Although harmonics have been generated up to 460 eV (order 183th or
2.7 nm) in He, the number of photons at such high energies—around 100
within a 1 % bandwidth of the continuous spectrum [73]—is too low for realistic
applications. In generel, there is a tendency for a slight decrease of the
harmonic yield with increasing order in the plateau region. On average, on
the order of 10 4 and 10 5 photons/pulse are typically generated in He and Ne,
respectively, for orders in the range 50–100 (78–124 eV) [46, 47, 53, 82]. For
Ar and Xe the corresponding numbers are 10 8 and 10 9 photons/pulse, respectively
for orders below the 25th (40 eV) with the Xe value corresponding
to a conversion efficiency on the order of 10 −6 [47, 53]. These values are consistent
with the numbers of 10 9 –10 11 photons/pulse stated for the 50–200 nm
region of four-wave mixing. The uncertainty on the above values is probably
around an order of magnitude, reflecting the scatter among the values
stated in the literature, and is due to differing experimental conditions and
the difficulties involved in absolute measurements. Setting a lower limit of
10 4 photons/pulse/0.1 % bandwidth for a realizable experiment, HHG should
be considered a useful technique for photon energies below ∼120 eV.
1.4 Comparison with other methods of VUV
light- and X-ray generation
As may be evident from the above, high-order harmonics constitute a useful
source of VUV light, competitive to synchrotron radiation for many
applications. Spatial [83] and temporal [84] coherence and automatic synchronization
with the generating laser field can be added to the properties
listed in the previous section. The limitations to the photon energies and
bandwidths that can be obtained at present do, however, justify a search
for alternative sources, especially for studies of structural dynamics where
synchrotron-radiation sources cannot provide the required temporal resolution.
A comparison of the presently available sources of VUV light and X-rays
for high-resolution studies in the frequency- and time domains are given in
tables 1.1 and 1.2, respectively. The listed parameters are only intended as a
guide, and in practice significant deviations may occur from the stated values.
For the synchrotron-radiation sources (ASTRID, ALS and SSRL) the
spectral bandwidth may be reduced at the expense of photon flux and still
be practically applicable. For example, the cross section of doubly-excited
states in He at ∼64eV has been measured at the ALS with a resolution of
1 meV corresponding to a 1.5×10 −5 relative bandwidth [85]. Also, as examplified
by the 1.4×10 −7 relative bandwidth obtained for 58.4nm radiation
by Eikema et al. (cf. section 1.1), harmonic radiation may be spectrally nar-