1A22
IR-VUV studies of 3-buten-2-ol monomer and multimers in a molecular beam.
(Tokyo Institute of TechnologyJonathan R. Woodward, Hirokazu Watanabe.
Mitsuhiko Miyazaki, Shun-ichi Ishiuchi and Masaaki Fujii[Introduction] Direct vacuum-ultraviolet (VUV) photoionization of molecules inmolecular beams with mass-spectrometric detection is a powerful complementary
technique to multiphoton ionization techniques like REMPI (Resonance Enhanced
Multi-Photon Ionization). It allows mass-selective spectroscopic measurements ofmolecules with inaccessible excited states or those possessing states with extremely
short lifetimes. By combining pulsed VUV photoexcitation with pre- or post- IR
excitation it is possible to record the infrared spectra of both ground state jet-cooled
molecules1 and ion-like Rydberg states2 with high spectral resolution and sensitivity.
Here we discuss the development of our instrumentation, its application to someexample model systems and its use in examining the conformational structure of 3buten-2-ol in a molecular beam and the structure and reactivity of its multimers.
[Instrumentation] VUV generation is achieved via a 4-wave mixing scheme in Kr orXe gas. Figure 1 shows the experimental layout of the spectrometer.

Figure 1. Experimental arrangement for the measurement of IR-VUV spectra of molecules in asupersonic jet expansion.

The VUV output of the laser, tuneable to an extent defined by the dye employed inlaser I, is focussed onto the molecular beam, and is free of any of the otherwavelengths employed in its generation. In this work, all VUV generation wasperformed with Kr gas, with phase matching achieved by tuning of the gas pressure.
In order to record photoionization threshold spectra free from any Kr re-absorption

features, small amounts of a low ionization potential calibrant molecule (triethylaminein this work) is also seeded in the molecular beam. In addition, this provides VUVpower calibration across the tuning region. The IR laser can access the spectral
region from approximately 2800 – 3800 cm-1; a useful region for the examination ofC-H, O-H and N-H stretching vibrations. IR-VUV spectra of molecular ground states
are recorded by adjusting the VUV wavelength to below the ionization threshold andsweeping the IR frequency, which is applied 50 ns before the VUV excitation.
[Experimental] 3-buten-2-ol is an ideal molecule for use in the study ofstereoelectronic effects due to the potential interaction between its neighbouring
alkene and alcohol moieties3. Previous studies have remained somewhat
inconclusive in determining which conformers are present in 3-buten-2-ol vapour atroom temperature. We have used VUV and IR-VUV photoionization methods toexamine this molecule and its ability to former multimers in a molecular beam. Figure
2 shows the mass spectrum
obtained by VUV irradiation of 3buten-2-ol at 129 nm. The 3buten-2-ol ion is clearly observed(and is free of fragmentation) alongwith an extended series of multimer
peaks. Of particular note is thesurprisingly broad signals observed
for the dimer and higher multimers.
This is suggestive of theirdecomposition within the mass
spectrometer. Intriguingly, mass
spectral signals are observed whichcorrespond to masses of 15 and 18 Figure 2. Mass spectrum obtained from the 129 nm
mass units less than the dimer irradiation of 3-buten-2-ol. The inset shows thesignal (mass =144), which appear ionization threshold spectrum of 3-buten-2-ol.
to correspond to the loss of methyl
and water respectively. Similar peaks appear corresponding to the trimer, but thetetramer shows only a peak corresponding to a loss of mass 15. Monomer anddimers show weak peaks corresponding to water cluster formation. The water
clusters become more prominent for larger multimers and for the octomer, the water
cluster signal is larger than the octomer itself. The inset shows the photoionizationthreshold spectrum of the monomer. We have recorded the IR-VUV spectrum of themonomer in the C-H and O-H regions and discuss the observed spectra incomparison with the results of DFT calculations. The threshold and IR-VUV spectraof the dimer are proving more difficult to successfully record. This difficulty may lie inthe decomposition process of the multimers, including dissociation from the larger
clusters. Experiments are underway to overcome these difficulties.
1. H. K. Woo, P. Wang, K.-C. Lau, X. Xing, C. Chang and C. Y. Ng; J. Chem.
Phys. 119 (8), 2003, 9333.
2. A. Fujii, A. Iwasaki, T. Ebata, N. Mikami, J. Phys. Chem. A. 101, 1997, 5963.
3. I. F. Shishkov, S Shlykov, B. Rousseau, Z. H. Peng, C. Van Alsenoy, H. J.
Geise, O. N. Kataeva, W. A. Herrebout and B. Van der Veken., J. Phys. Chem.
A, 105, 2001, 1039.