J. Phys. Chem. 1992, 96,4821-4824
4821
Detection and Spectroscopic Studies of Gas-Phase OH-Kr by Laser-Induced Fluorescence George W. Lemiref and Rosario C . Sausa*
US.Army Ballistic Research Laboratory, SLCBR-IB-I, Aberdeen Proving Ground, Maryland 21005-5066 (Received: January 24, 1992; In Final Form: March 4, 1992)
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The van der Waals complex OH-Kr has been detected in a free jet expansion by laser-induced fluorescence (LIF) near the (1,O) band of the AZZ+ X2n,transition of uncomplexed OH. The excitation spectrum reveals a progression of vibrational bands which have been assigned to the stretching motion of the complex. A Birge-Sponer plot of this progression yields a fundamental frequency of w,’ = 196 8 cm-l and an anharmonicity constant of w,’xc) = 7.4 f 0.7 cm-l for the A2Z+ excited state. The lower limit of the binding energy of the complex is measured as Dgl(u’=l) 2 1131 cm-l. These results are compared to those obtained from recent gas-phase LIF studies on the OH-Ne and OH-Ar complexes near the A22+ X2ni(0,O)and (1,O) bands of OH, respectively, and those from a matrix LIF study on the OH-Kr complex near the (2,O) band of OH.
*
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Introduction In recent years there has been a considerable interest in open-shell, rare gas diatomic van der Waals (vdW) systems. Unlike their closed-shell counterparts, only a few systems have been detected spectroscopically. These include, for example, nitric oxidc-argon’” (NO-Ar), hydro~yl-argon”~~ (OH-Ar), and the more recent hydroxyl-neon2031(OH-Ne). The interest is in part due to the novelty of these species and the experimental and the~retical~ opportunities ~-~~ they provide in the understanding of collisional dynamics and structure of weakly bound species. The interaction of OH or NO with other collision partners is also of significance in combustion and atmospheric chemistry. The first observation of hydroxyl-krypton (OH-Ar) vdW complexes was made by Goodman and Brus8 in cryogenic noble gas matrices. Their investigation was concerned primarily with the understanding of hydrogen bonding. The complexes were detected by laser-induced fluorescence (LIF) near the OH A2Z+ X211iultraviolet system and with wavelengthdispersedemission. A series of progressions were observed in the excitation spectra near the OH (2,0), (l,O), and (0,O)bands and assigned to the vdW stretching motion. The observed emissions were broad continua, red-shifted from the OH bands. Although most of their quantitative data dealt with OH-Ar and OD-Ar complexes, one progression in the OH-Kr complex was reported near the (2-0) band of OH. The bands in this OH-Kr progression were so broad that only the band positions were reported. As a result of their observations, Goodman and Brus determined that the upper electronic state of the OH and OD rare gas complexes have significantly higher binding energies and shorter vdW bond lengths than the ground state. Goodman and Brus’s paper has prompted numerous gas-phase studies of OH-Ar.’-19 The complexes generated in these studies were the result of free jet expansions and detected by LIF. The focus of much of this previous work has dealt with determining the geometries and describing the intermolecular potentials of the ground and excited states of this complex. Rotational analyses of many of the stretching bands in the OH-Ar complex have been shown to be consistent with a linear rigid rotor model even under high resolution (250 MHz).I9 In describing the intermolecular potentials, Berry and c ~ - w o r k e r s ~ have ~ - ~studied ~ the predissociation dynamics of excited vibrational levels in both the excited electronic and ground states. In this paper, we report the detection of the OH-Kr vdW complex in the gas phase by LIF in the region of the uncomplexed OH A2Z+ X2ni(1,O) band near 281 nm. A series of progressions were observed and assigned to the vdW stretching motion of the complex. The binding energy D,,’ of the complex in the +
+-
’BRL/NRC Postdoctoral Research Associate. To whom correspondence should be addressed.
OH AZZ+(J=1) state together with its fundamental frequency and anharmonicity constant w d x l are obtained from a 2rge-Sponer analysis. A comparison between these results and the matrix work of Goodman and Brus on OH-Kr near the A%+ X2ni(2,O) OH band and recent gas-phase work on OH-Ne and OH-Ar near the OH (0,O)and (1,O) bands, respectively, is made.
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Experimental Section The experimental apparatus used in these experiments is a general purpose molecular beam apparatus that has been constructed for both laser-induced fluorescence and resonance-enhanced multiphoton ionization (REMPI) experiments of photolytically generated species. A schematic of the apparatus configured for LIF studies is presented in Figure 1. There are two main chamber bodies to this apparatus. The first chamber consists of an 8-in. Tee with smaller ports added for laser diagnostics, fluorescence detection, and pressure recording. This chamber is pumped out through the bottom by a 1000 L/s turbo molecular pump (Leybold-Heraeus, TurboVac 1000). At the rear of the Tee is a pulsed supersonic valve (R.M. Jordan Co., PVS) that can be translated the length of the chamber and can be positioned for either LIF or REMPI experiments. At the front of the Tee is mounted an &in. four-way cross, not shown in Figure 1, serving as the second chamber for time-of-flight mass spectrometric studies. The OH-Kr and OH-Ar vdW complexes were produced by photolyzing acetaldoxime [CH,(H)CN-OH] (Aldrich Chemical Co., 99%) vapor seeded in krypton or argon (Spectra Gases Inc., 99.995%) gas, respectively, in an extender channel prior to supersonic expansion. Acetaldoxime has been shown to be a good source of OH radicals when photolyzed with UV r a d i a t i ~ n . The ~~ experimental cycle was limited to 10 Hz by the pulsed supersonic valve which was also operated with a backing pressure of 100-140 psi. The acetaldoxime was photolyzed in the peak of the gas density by focusing 5-10 mJ of 193-nm radiation from an excimer laser (Lambda Physik, EMG 15OMSC), collinear to the molecular beam, with a 1-m lens into the extender channel. The extender channel (0.5-0.7-mm diameter and 1-cm length) was mounted directly to the front of the pulsed valve. The OH-Kr and OH-Ar vdW complexes were probed perpendicular to the molecular beam 3-5 mm in front of the extender, 6-8 w after the photolysis pulse. Laser-induced fluorescence signals were collected normal to the plane defined by the photolysis and probe laser beams. The probe laser used was a XeCl excimer pumped dye laser system (Lumonics Hyper EX-400 and Hyper DYE-300) that was frequency doubled (Lumonics TRAK-1000) in order to generate tunable UV radiation in the region of 2800-2900 A (coumarin 540A). The line width of the fundamental frequency of the dye laser is -0.08 cm-l (fwhm) while that of the doubled frequency is -0.16
This article not subject to U S . Copyright. Published 1992 by the American Chemical Society
4022 The Journal of Physical Chemistry, Vol. 96, No. 12, 1992
Lemire and Sausa
E MONOCHROMATOR
2.0
5i ZJ
'0
i5
1.5 1.0 0.5
EXCIMER LASER
0.0
Xecl (308m)
7
POWER
1
cm-l (fwhm). The OH-& vdW complex was excited in the same spectral region as the OH-Ar c o m ~ l e x ~near " ~ ~the uncomplexed OH A2Z+ X211i (1,O) band. The fluorescence was focused on the entrance slits of a 0.25-m monochromator (McPerson, Model 218) tuned to approximately 309.5 nm and used as a broad-band filter (-8-nm fwhm). No other filters were used. The signal was detected with an ungated photomultiplier tube (EM1 9789QA), directed to a gated integrator (Standard Research Systems), and displayed on a 350-MHz digital oscilloscope (LeCroy 9420). In general, the voltage required on the photomultiplier tube for detecting the OH-Ar and OH-Kr complexes were 1650 and 1150 V, respectively (a gain difference of approximately 100). The spectra were recorded in digital form on a PC-AT computer via a commercial interface/systems package (Stanford Research Systems).
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RCSults Laser-induced fluorescence excitation scans were recorded in the region of both the (0,O)and the (1,O) bands of the OH A2Z+ X211i(0,O)system in search for spectra of the OH-Kr van der Waals complex. The only spectra observed were those obtained in the region of the (1,O) band of OH near 281 nm. One of these is presented in Figure 2. For comparison, a LIF excitation spectrum of the OH-Ar complex was also recorded in the same spectral region under identical conditions except for carrier gas. The observed features are similar to that reported by Berry and co-~orkers.'~The three strongest features in Figure 2, labeled P( 1S), Q( 1S), and R( 1S), are actually due to the uncomplexed OH A2Z+ X2nj(1,O) transition. In fact, all of the sharp lines observed in the spectrum are part of the OH system. The presence of these lines results from higher gain settings in the photomultiplier tube and not higher rotational temperatures as would be expected in a supersonic expansion. When the gain on the photomultiplier tube is turned down to the level of that used in the OH-Ar experiments, only the strongest features, P( 1S), Q( 1S), and R(1.5), remain. The emission monitored following 28 1-nm excitation was centered around 309.5 nm. It is attributed to the (0,O)transition of the OH 2Z+(v'=O) fragment formed from a rapid vibrational predissociation of the complex in the 2Z+(v'=1) excited state. A similar occurrence has been reported for the OH-Ar complex when
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I
I
I
I
I
I
2830
2840
2850
2860
2870
2880
(A)
Figure 2. Laser-induced excitation spectrum of the OH-Kr vdW complex observed in the region of A2Z+-X211j(1,O) transitions near 28 1 nm. The fluorescence was monitored around 309.5 nm.
Figure 1. Schematic of the experimental apparatus configured for LIF experiments (top view).
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I
2820
WAWLENGTH
LENS
BEAM
I 2810
TABLE I: Positions ( v ) and Spacings (AG) in Units of cm-' of Experimentally Observed OH-Kr Spectral Features Assigned to tbe vdW Stretching Progression near OH A22? X2n, (1,O) and (2,O) Transitions matrix work" present workb OH-Kr (2,O) OH-Kr (1,O)
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U
V
36 686 36 896 37 074 37 226 37 365 37 470 37 564 37 661
AG
V
AG
21v 178 152 139 105 94 96
34 325.7c 34 5O6Se 34 672.4 34 823.5 34 959.6 35 082.0 35 190.4 35 276.0 35 356.3
180.8d 165.9d 151.1d 136.1 122.4 108.5 85.6 80.3
"Reference 8. Values of v were reported in units of angstroms and were converted to wavenumbers (vacuum). bLine width of laser is approximately 0.2 cm-'. Possible transcription error was found for this value in the above reference. Value was reported as 215 cm-'. dValues obtained from a BirgeSponer extrapolation. e Values obtained by subtracting AG's from the first observed member of the progression.
excited near the OH (1,O) band.14J5 If the complexes did not predissociate, their fluorescence would be expected to occur in the region of the uncomplexed OH (1,l) transition near 3 15 nm. When this region was monitored, no features other than those resulting from uncomplexed OH transitions were observed. The spectra of the vdW complexes were simplified since the interference of OH lines resulting from the (1,l) transition near 315 nm was minimal when monitoring the 309.5-nm emission. However, the emission was too weak to be dispersed with a monochromator. Under our experimental conditions, the complexes would have probably not been observed if they did not predissociate since their fluorescence signals would have been. overshadowed by the much stronger OH signals resulting from the (1,l) transitions. In fact, it is probably this reason that no features of the OH-Kr complex were observed when exciting it near the (0,O)band of OH. Table I contains a list of the band positions assigned to the stretching mode in the vibrational progression of OH-Kr, labeled 3-8, in Figure 2. Additional features in Figure 2, labeled 3b-6b and A-D, are assigned analogous to those reported for the OH-Ar c0mp1ex.l~ The features labeled lb-6b are slightly red-shifted relative to the main OH-Kr progression and are attributed to a larger vdW OH-Krx specie. In the OH-Ar spectrum, the features labeled A-D, also referred to the U system,17involve some degree of mixing between the bending and stretching modes, and as a result they do not fit a simple vibrational progression?2 Therefore,
The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 4823
Spectroscopic Studies of Gas-Phase OH-Kr
TABLE Ik Vibrational Const.nts in Units of cm-' for the OH-Kr Stretching MOW w,I = 196
wcx,I = 1.4
c
'E 0
D[ = 99b D[ 2 28'
Dd = 1202b
DO'1 1131'
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Parameters were determined from spectral features of the complex associated with excitation of the OH A22+ X2n, (1,O) band near 281 nm. bValues obtained by using the equation DO' = ( u i u , ' x ~ ' ) / ~ w : x : . 'These values are lower limits of the binding energies of the complexes imposed by the position of feature D in the (1.0) region.
n F
+
2 W
AC(u+l) intercept for the OH-& complex yields a value of 196 f 8 cm-'for the fundamental frequency uc),while the slope yields a value of 7.4 f 0.7 cm-' for the anharmonicity constant w:x,I. These values are the first to be reported and thus cannot be compared. However, if these values are compared to those obtained for the OH-Kr near the A22+ X2ni(2.0) OH band in the matrix work, then w,I would be smaller by 39 cm-' and w,Ix,' by approximately 2.6 cm-I. A similar trend was found for the Ar-OH complex. In particular, the gas-phase value^'^^'^ of :a and LO,'&for ' the J = 1 excited state are smaller by approximately 29 and 3.0 cm-', respectively, when compared to the matrix values for the u' = 2 excited state. A linear extrapolation of the BirgeSponer plot yields a binding energy of Do' = 1202 cm-' for the A%+ (u'=l) excited state of OH-Kr. A lower limit of Dd(v'=l) L 1131 cm-l is obtained basing the binding energy on the last observable feature (D). This value is consistent with the lower limit value of lo00 cm-'reported for the u' = 2 state in a matrix. For comparison, the lower limit value for the binding energy of OH-Ne is reported20as 61.8 cm-' while those for OH-Ar fall in the of 675-755 cm-I. The ground-state binding energy (Do") of OH-Kr can be calculated using the equation D,"(OH-Kr,u"=O) - D
