UV and VUV absorption and emission spectroscopy is used to probe jet cooled molecules, free radicals, and clusters in the gas phase. Due to efficient cooling inhomogeneous effects on spectral line widths are eliminated. Therefore from these spectra, both structural and dynamical information is obtained. The photoproducts of these reactions are probed by resonance enhanced multiphoton ionization

Ruehl, E.

Vaida, V.

English

NASA Technical Reports Server

N90-26771
UV AND VUV SPECq_OSCOPY AND PHOTOCHEMISTRY OF SMALL
MOLECULES IN A SUPERSONIC JET
E. RU'HL* AND V. VAIDA
Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309
*permanent address: Institut f'tir Physikalische und Theoretische Chemie, Freie Universitht
Berlin, Takustr. 3, D-1000 Berlin 33, Federal Republic of Germany.
ABSTRACT
UV and VUV absorption and emission spectroscopy is used to probe jet cooled molecules,
free radicals, and clusters in the gas phase. Due to efficient cooling inhomogeneous effects on
spectral line widths are eliminated. Therefore from these spectra, both structural and dynamical
information is obtained. The photoproducts of these reactions are probed by resonance enhanced
multiphoton ionization.
INTRODUC_ON
In the last years an experimental research program has been developed in our laboratory
which allows study of reactive small molecules, radicals, and their molecular aggregates. This
program employs a combination of supersonic molecular beams for sample preparation with
spectroscopic and photofragrnent techniques, used to characterize these reactive species.
RESULTS AND DISCUSSION
Jet cooled molecules such as NH3, CS2, OCS, and CH3I have been studied during the last
years by vacuum UV spectroscopy. 1-3 These improved spectra provided structural and
dynamical models for the excited states of these small reactive molecules. Besides absorption
features known for the isolated molecules, new absorption bands have been observed at high
stagnation pressure. These bands have been assigned as dimer absorption bands. The
experimental scope has been extended recently to high resolution UV spectroscopy in order to
understand monomolecular reaction dynamics of photoreactive systems. This is accomplished
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by usinga Fouriertransformspectrometer(FTS)with a free jet expansion. Compared to the best
conventional free jet absorption technique, 1 the FTS offers a 15 times greater sensitivity and two
orders of magnitude better spectral resolution. This technique has been applied to study reactive
electronic states of chlorine dioxide (OC10). 4 Upon electronic excitation (_,(2A2)--- X(2B 1), 260-
480 nm) OC10 predissociates into the fragments C10 + O. With rotational cooling in a
supersonic expansion spectral congestion due to inhomogeneous effects can be reduced
substantially. From the rotational envelopes of the vibrational bands a rapid increase in the line
widths is observed as the energy of the transition is increased. Figure 1 shows a portion of the jet
cooled (Tk(2A2)---_K(2B 1) absorption band of chlorine dioxide. The splittings in each band are
caused by the 35C1 and 37C1 isotopes.
io
I0
_(¢= .I)
Figure 1: Portion of the jet cooled absorption spectrum of chlorine dioxide. Recorded at 5cm -1
resolution and 2 atm total stagnation pressure (3% OC10 in He).
The Fourier transform technique can be also used for emission spectroscopy. This has
been demonstrated for emission of jet cooled CN(B 2E) radicals.5 The spectroscopic sample is
formed in a continuous corona discharge. The B---X emission spectrum of CN is shown in
Figure 2. The advantage of the corona discharge radical source is that high vibrationaUy excited
levels can be studied in a relatively uncongested rotationally cold spectrum. 6 From the spectrum a
vibrational temperature of 2200 K and a rotational temperature of 75 K has been deduced. New
and more accurate rotational constants for the CN B-state have been obtained by the use of K
values up to 62.
256
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Figure 2: CN (B--*X) emission spectrum recorded at 0.25 cm -1 spectral resolution under
stagnation conditions of 0.13 atm CH3CN in 1 atm He. The spectrum includes both the 0-0 and
1-1 transitions. Arrows indicate perturbations from A-state rotational levels.
Besides spectroscopic probes we have employed mass spectrometry to analyze the
products of reactive electronic states of jet cooled molecules. The technique used is resonance
enhanced multiphoton ionization (REMPI). Chlorine dioxide has been studied in the spectral
regime of (A(2A2)---X(2B1) transition. 7 The main products of the above mentioned
predissociation is C10 + O. Chlorine oxide is probed in a (2+2) REMPI process, in which the
C(2E)--(2I-[) is accessed (see Figure 3a). All REMPI bands observed are due to vibrationally
excited C10 (X(21-I)(v=3-6)). The rotational profiles of the REMPI bands corresponds to a
thermal Boltzmann population with T=100 K. At higher photon energy (around 340 nm)
additional rotational fine structure of vibrationally excited CIO (A(21-I)---X(2I-I)) occurs in the
REMPI spectrum of C10 +. Besides predissociation of OC10 we find evidence for
photoisomerization of OC10 (_,(2A2)). A single resonance in the C1+ REMPI signal occurs at 362
nm (see Figure 3b). This agrees with matrix work on chlorine dioxide, where around 365 nm
photoisomerization into C1OO was observed.8
Due to its low binding energy C1OO decays into C1 + 02. Because of the low multiphoton
ionization cross section of molecular oxygen we observe only a C1 + REMPI signal which
suggests photoisomerization. We have recently discussed the potential importance of OC10
photoisomerization with respect to polar ozone depletion. 9
257
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CIO+
i i J i i i i
' ' I .... I .... I
355 360 365
;L/nm
Figure 3: Resonance enhanced multiphoton ionization (REMPI) spectrum of C10 + [a], and C1+
[b] between 353 and 369 nm.
Currently we are extending our research program to small predissociating molecules which
are of interest for planetary atmospheres, such as HCN, H2S, and H20. We are also interested in
the spectroscopy and photoreactivity of homogeneous and heterogeneous clusters of these
species. Because cluster formation is enhanced at low temperatures, their photoreactivity might be
important to understanding the chemistry of the atmospheres of outer planets.
ACKNOWLEDGEMENTS
This work is supported the National Science Foundation and Petroleum Research Fund.
E.R. acknowledges financial support by Deutsche Forschungsgemeinschaft.
258
REFERENCES
V. Vaida, Acc. Chem. Res. 19, 114 (1986)
V. Vaida, M.I. McCarthy, P.C. Engelking, P. Rosmus, H.J. Werner, and P.
Botschwina, J. Chem. Phys. 86, 6669 (1987); P. Rosmus, P. Botschwina, H.J.
Werner, V. Vaida, and P.C. Engelking, J. Chem. Phys. 86, 6677 (1987); M.I.
McCarthy, P. Rosmus, H.J. Werner, P. Botschwina, and V. Vaida, J. Chem. Phys. 86,
6693 (1987).
V. Vaida, D.J. Donaldson, S.P. Sapers, R. Naaman, and M.S. Child, J. Phys. Chem.
93, 513 (1989).
E.C. Richard, C.T. Wickham-Jones, and V. Vaida, J. Phys. Chem. 93, 6346 (1989).
E.C. Richard, D.J. Donaldson, and V. Vaida, Chem. Phys. Lett., 157, 295 (1989).
P.C. Engelking, Rev. Sci. Instr. 57, 2274 (1986).
E. Riihl, A. Jefferson, and V. Vaida, J. Phys. Chem., submitted (1989).
A. Arkell and I. Schwager, J. Am. Chem. Soc. 89, 5999 (1967).
V. Vaida, S. Solomon, E.C. Richard, E. Riihl, and A. Jefferson, Nature, in press
(1989).
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UV and VUV spectroscopy and photochemistry of small molecules in a supersonic jet

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