The Journal of
Physical Chemistry
0 Copyright 1993 by the American Chemical Society
VOLUME 97, NUMBER 45, NOVEMBER 11, 1993
LETTERS Doppler Polarization Spectroscopy of the Photofragments from an In-Plane Rotation of Water: Demonstration of Unperturbed Vector Correlations D. David, I. Bar, and S. Rosenwaks' Department of Physics, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel Received: August 2, 1993; In Final Form: August 30, 1993'
The p v , v-J, and p-v-J vector correlations of the OH fragments ejected from the photodissociation at 193 nm of the 303 in-plane rotational state of the fundamental symmetric stretch of water, H2O (l,O,O), are probed by Doppler polarization spectroscopy. The observed correlations are close to the limiting values expected for an idealized orientation (with minimal influence of the parent rotation) where the transition dipole moment of the parent is parallel to the fragment angular momentum and perpendicular to its velocity. This is the first time that these correlations are measured for the photodissociation of an initially prepared, vibrationally excited molecule in a specific rotational state.
Introduction
-
-
The photodissociation of water in the first absorption band
Insight into the mechanisms underlying molecular photodissociation is obtained by determining both the scalar and the vector properties of the photofragments. The vector properties are monitored via Doppler and polarization spectroscopy'" utilizing the preferential absorbance of light by molecules whose transition dipole moments, p , are aligned parallel to the electric vector of the linearly polarized photolysis laser radiation, ep. In rapid dissociation, the alignment of p may lead to recoil anisotropy (in the laboratory frame) of the photofragments, Le., of the distribution of the product velocity vectors, v, relative to tp (the so-called p-v correlation). In addition, fragment rotational alignment may be obtained as a result of an anisotropy in the distribution of the fragment angular momenta, J, relative to tp and thus t o p (the CCJ correlation). Also, since both J and v are correlated with p , a mutual correlation between v and J (the v-J correlation and the triplet p v - J correlation) may result. The correlations that can be extracted from the polarized Doppler broadened spectral line shapes and from time-of-flight spectra of the fragments shed light on the motion of the photofragments in the molecular and laboratory frame and lead to an understanding of the photodissociation dynamics.'" Abstract published in Advunce ACS Abstracts, November 1 , 1993.
H,O(X)
+ hv
H20(A'Bl)
H('S)
+ OH('II)
(1)
is a prototype of a fast and direct bond rupture in an excited electronic state, and therefore considerable experimental7-11 and theoretica17J2work devoted to the dynamics of the photodissociation was carried out. Thus, absorption spectroscopy, finalstate distributions of the products, photodissociation of vibrationally excited states, isotope effects, and emission spectroscopy were inve~tigated.~Although many aspects of the photodissociation of water in the first absorption band have been studied, very few experiments were conducted toward the elucidation of the vector properties. In previous works on these properties, vibrational ground-state water molecules were photodissociated. Andresen et ale8measured a higher degree of rotational alignment for cooled molecules than for molecules photodissociatedat room temperature. Comes and co-workers9 monitored the Dopplerbroadened line profiles of the OH fragments following photodissociation at 193and 157nm; they concluded that the fragments are ejected perpendicular top and determined their recoilvelocity. Recently, we have determined the rotational alignment, Ai2),of the OH fragments obtained following photodissociation of the symmetric stretch vibration, H2O (1,O,O),from different initially prepared rotations.11 However, polarized Doppler profiles, at
0022-3654/93/2097-11571$04.00/00 1993 American Chemical Society
Letters
11572 The Journal of Physical Chemistry, Vol. 97,No. 45, 1993 different laser geometries for different OH transitions,from which the various vector correlations can be determined, have not yet been measured, and therefore the complete picture of the fragmentation is still not available. In particular, as stated by Hall and H o u ~ t o n ,no ~ one has yet tested how the vector correlations are affected by selection of the initial rotational level of the parent molecule. This kind of measurement is essential since parent molecular rotation has been suggested as one of the reasons for decrease in the values of vector correlations.ld In this Letter we report on the p-v, v-J, and p-v-J correlations of the OH fragments photoejected at 193 nm by water molecules initially prepared in an in-planerotational state of the fundamental symmetric stretch, H20 (1,O,O). An in-plane rotational state has been chosen since it was anticipated that it would not affect the correlations, and therefore the observed values would directly reflect the photodissociationdynamics. The observed correlations are indeed close to the anticipated limitingvalues for an idealized orientation of p parallel to J and perpendicular to v. The dynamical aspects of the photodissociation of water are discussed in view of these observations. Experiment
The experimental approach and the apparatus have been described in detail elsewhere.I0JI Briefly, stimulated Raman excitation (SRE) and coherent anti-Stokes Raman scattering (CARS) are used to prepare and detect, respectively, particular rotational states of H20 (l,O,O). The excitation is effected with two visible beams, the second harmonic of a Nd:YAG laser at 532 nm (- 50 mJ) and a tunable dye laser pumped by it, operating at -660 nm ( - 5 mJ). Both beams are parallelly polarized, collinearly combined, and focused into the photodissociation region. The beams are precisely overlapped spatially and temporally. The frequency difference of the two lasers corresponds to excitation of the 303 rotational state of H20 (1,0,0) through the isotropic Q-branch transition.” This transition is chosen since it is strong and well resolvedIOJ3 and prepares the H20 molecule in a pure in-plane rotational state.I4 The resulting CARS signal (which measures the SRE excitation and allows its optimization) is monitored by a photomultiplier tube (PMT) equipped with filters and imaging optics. Monitoring the CARS signal enables to assure constant level of excitation during the experiment. I Following the excitation, after 25 ns, the rovibrational excited molecules are photodissociated by a 193-nm beam from an ArF excimer laser (- 1 mJ). The photodissociatingbeam is polarized by a quartz double-prism Rochon polarizer (which allows the use of either P (0-ray) or S (e-ray) polarization direction, cp) and then enters the reaction cell counterpropagating the SRE beams. The frequency-doubledoutput beam of a Nd:YAG-pumped dye laser around 282.4 nm is used for laser-induced fluorescence (LIF) detection of the OH fragments on the Q1(4) and Pl(4) lines of the A, u’ = 1 X,u” = 0 transition,lS after an additional delay of 25 ns. The bandwidth of the probe laser is narrowed to -0.08 cm-1, in the UV, with an intracavity air-spaced angle-scanned etalon. The probe beam passes through an Archard-Taylor polarizer and a X/2 retarder (two Fresnel rhombs) which allows rotation of its polarization direction, ea. The photolyzing and probe beam geometries could be altered to allow parallel or perpendicular alignment of the polarization (tplltar c p l ea). The probe propagation direction (ka) is colinear with the SRE beams, thus counterpropagating the propagation direction of the photodissociating laser (kp). The pulse energy of the probe laser is attenuated to -0.2 pJ in order to preclude saturation of the OH LIF signal. The LIF signal is detected at a right angle to the laser beams with a PMT equipped with an interference filter (314 f 5 nm) and imaging optics that is insensitive to the polarization. The signal from the photodissociation of the 303 rotational state of H2O (1,0,0) is accompanied by a small
-
background from the photodissociation of vibrational groundstate molecules. Water vapor is flowed at pressure of 100 mTorr to minimize collisional relaxation of the initially prepared HzO state and of the rotationally excited OH fragments.1°
Data Analysis The 303 rotational state of H20 is prepared via SRE using parallel polarized beams, and it is therefore necessary to check whether this leads to any degree of alignment of this state. Such alignment, if exists, would affect the subsequentphotodissociation dynamics and alter the analysis of the vector correlations. However, as explained in ref 11, the transitions utilized for preparation of H2O (1,O,O)belong to the isotropic Q-branch, and therefore any initial alignment (if it exists) is negligible, allowing the utilization of theusual vector correlation treatments of Greene and Zarel6 and of Dix0n.l’ The correlations among p , v, and J affect the fragment Doppler profiles.17.18 Assuming single speed for the fragment, the profiles are convoluted with the parent thermal velocity distribution and the probe laser bandwidth which can be represented by a combined Gaussian function. The expression to be fitted to the observed profile is given by17J9
where denisthe effectiveanisotropyparameter, P ~ ( xis ) the second Legendre polynomial, and 0 is the angle between ep and ka (0 = 90° in our experiments); YO and AVDare the line center and the maximum Doppler shift, respectively; va represents the probe laser frequency, and Av, is the width of the Gaussian convolution function. Combining the 0.08-cm-I bandwidth of the probe with the 0.103-cm-’ parent thermal distribution width at 300 K gives an overall full width at half-maximum (fwhm) of Avr = 0.13 cm-1. The optimized parameters in the least-squaresfit procedure are AVD,the amplitude A, the background E, and pen. The value of Ben is related to the bipolar moments $(klk2) that describe the vector correlations:I9
The moments &(20), &02), &(22), and &22) describe the v-J, and p - V J correlations, respectively; b& are angular momenta and geometricalcoupling factors.17 The bipolar moments are deduced from the values of perfdetermined from the LIF transitions, Le., the Ql(4) and P1(4) lines, in the different experimental geometries. p-v, p J ,
Results and Discussion Shown in Figure 1 are the observed Doppler profiles for the Ql(4) (panela)andPl(4) (panel b) LIFtransitionsofOHresulting from the photodissociation of the 303 rotational state of HzO (1,O,O)as obtained in the two geometries (A and B) shown at the top of the figure: c a l c p and callep; in both cases the detection is along the Y axis. The solid lines show the best fit of the profiles to the expression given in eq 2; the residuals are presented below the profiles. The fwhm of the profiles, 0.274 cm-1, is mainly due to the Doppler shift (its contribution to the fwhm is ~ A v Dwhich ), is larger than the width of the Gaussian convolution function, Ava. The deduced value of AVD,0.121 f 0.004 cm-1, corresponds to a fragment recoil velocity of 1025 f 34 m s-1. Based on conservationof energy and linear momentum? the recoil velocity
The Journal of Physical Chemistry, Vol. 97, No. 45, 1993 11573
Letters
B
A
I "k
-0.8
-0.4
0
Avlcm'
'
014
'
018
-0.8
-0.4
'
0
0.4
C
Figure 1. OH photofragment Doppler resolved profiles (dots) of (a) Ql(4) and (b) p1(4)transitions in the two geometries shown at the top, resulting from photodissociationof the 30, rotational state of H20 (1,0,0) at 193 nm. The fits (solid lines) and residuals (below the profiles) are obtained using eq 2.
TABLE I: Observed Bipolar Moments and Limiting Values for the Idealized Orientation of rrllJlv' Oh3CrVed limiting values for
4.44 -0.5
0.15
-0.42 f 0.22 4.5
+0.34
+os
* 0.20
PPLv
The moments are deduced fkm the least-squares solution of the set values obtained for the of simultaneous equations (3) utilizing the Doppler profiles shown in Figure 1 and the value of the rotational alignment, A&z) 0.43 i 0.15, from ref 11. a
-
is estimated to be 1045 m s-l, which agrees very well with the observed value. From the differences observed in the shapes of the profiles (see Figure l), a set of four values of Benis obtained: -1.1 2 and -1.46 for Q1(4) and -0.70 and 0.00 for Pl(4) in geometries A and B, respectively. The estimated uncertainties for the values of Ben, based on the sensitivity of the fitting procedure, is *0.01. This set of values is employed in a least-squares solution of the four simultaneous eqs 3, along with the value of &02) obtained from the rotational alignment, Ai2) (Bi(02) = 5/4Ah2)).17The latter measured from the integrated intensity ratio of Q lines in two experimental geometries.11 Using the value measured for Q1(4), Ah2)= 0.43 f 0.15,the bipolar moments are extracted and given in Table I. As shown in the classical description of the photodissociation of water presented in Figure 2, the transition dipole moment 1 for the 193-nm A X excitation is perpendicular to the molecular
-
Figure 2. Schematic view of the classical description of the photodis-
sociation of water including the p-v, p-J, and J-v correlations. Also shown are the experimental geometries including the directions of the PMT and of the laser propagation (k&) and polarization (e&,, $le,) vectors. The full circles represent 0 atoms and the empty circles H atoms. plane.7v8J2 This statement is alsovalid for photodissociation from the lowest vibrational levels of the X state.l2 Hence, molecules aligned perpendicular to the direction of e,, are preferentially dissociated. In a complete planar dissociation the rotation and recoil velocity of the OH products are in the H20 plane. As a result of the planar dissociation, the rotation and recoil velocity of the OH products are in the H20 plane. As a result of a planar dissociation, the relative orientations of the vectors are Jill, vlp, and as a result J l v , as shown in Figure 2. The limiting values of the bipolar moments expected for the idealized orientation17 of p1JJJ-vat high J are given in Table I. The observed moments approach the limiting values, suggesting that the dissociation is
11574 The Journal of Physical Chemistry, Vol. 97, No. 45, 1993
very close to being completely planar. Also shown in Figure 2 are the experimental geometries including the directions of the detection (PMT) and of the lasers propagation (k,Jlk,) and polarization (tJta, e p l t p ) vectors. Parent molecular motion before and during the dissociation process may result in deviation of the observed vector correlation parameters from their limiting values. The influence of parent rotation on the observed alignment and on the "usual" anisotropy parameter, j3 (j3 = 2&(20)), has extensively been treated, e.g., refs 20 and 21, respectively. Also, we have demonstrated that in photodissociation of water different types of parent rotation affect the fragment alignment." It was found that parent inplane rotation has no influence on the observed alignment while out-of-plane rotations result in a decrease of the alignment. The fact that in the present experiment the observed correlations approach their limiting values indicates that, for these correlations as well, an in-plane rotation preserves the planarity of the dissociation. Although the deviations of the measured correlations from the limiting values lie within the estimated uncertainties, all the measuredvaluesseem to be lower. Apparently, the slight deviation of the correlations that involve J from the limiting values is mainly due to the low J(J = 4.5) at which they have been measured. The high-J limit for OH is achieved for J > 7 S S 8 Similar deviation also was observed in our alignment measurements." For an inplane rotation the main reasons for the deviations at low J is the A-doublet mixing and the hyperfine structure of the OH.sJl Busch and WilsonZ2presented a simple model for the dependence of j3 on parent configuration, rotation, and excited-state lifetime in triatomic molecular photodissociation. In this model the direction of v relative to p might be deflected due to the parent rotation. It can be concluded from the model that for p l v no deflection should appear for parent in-plane rotation. This conclusionis confirmed by the present results that show that the p-v correlation approaches the limiting value. The effect of parent vibrational motion on /3 in triatomic molecular photodissociation has recently been treated by Loock et al.23 They have found that parent bending motion on the excited-state potential can significantly change the value of 0. However, the vibrational influence affects only in-plane transition moments,23and thus noinfluenceof the initialvibration is expected in our experiments. The slight deviation of the observed p-v moment from its limiting value may be a result of the small background signal from the photodissociation of tibrational ground-state molecules, H20 (O,O,O). These molecules are rotating (about equally) in and out of plane and are thus described by a different anisotropic parameter, denoted hereafter as @back. The decrease in j3kck from the limiting value depends on the temperature and the excitedstate lifetime, 7.21922 Unfortunately, the exact lifetime is not known, and only a rough estimate can be made. Following the model of ref 22 and assuming T = 300 K and 7 = (2-4) X lO-I4 5,s the value of j3bck is -0.70 to -0.45, respectively. These values are lower than the observed value, = -0.88 f 0.30. Taking intoaccount the weighted backgroundc~ntributionl~ and assuming a limiting value of j3 = -1 for photodissociation of the 303 state
Letters of H20 (l,O,O), the expected value of j3 can be calculated. We estimate that the signal from the photodissociationof Hz0 (1,0,0) is 5-6 times larger than the background signal; therefore, j3 is expected to be -0.92 and -0.95 for 7 = 4 X 10-14 and 2 X l@I4 s, respectively. These values are close to Bobs, implying that the main reason for the deviation from the limiting value is indeed the background signal. To summarize, the observed correlations areclose to the limiting values expected for an idealized orientation where the transition dipole moment of H2O (l,O,O) is parallel to the OH fragment angular momentum and perpendicular to its velocity. This is in line with the suggestion of Schinke and co-workers12 that p is perpendicular to the molecular plane for the lowest vibrational statesof the water molecule. By preparing a molecule in a selected rovibrational state, it might be possible to minimize the influence of parent rotation on the vector correlations and to extract the vector properties resulting directly from the dissociation dynamics. This method can be extended to measurements of dissociation from different rovibrational states and thus allows one to examine how the selection of an initial state affects the vector correlations.
References and Notes (1) Ashfold, M. N. R.; Lambert, I. R.; Mordaunt, D. H.; Morley, G. P.; Western, C. M. J. Phys. Chem. 1992, 96, 2938 and references therein. (2) Siebbeles, L. D. A.; Beswick, J. A. J . Chem. SOC.,Faraday Trans. 1992, 88, 2565. (3) Dixon, R. N. Acc. Chem. Res. 1991, 24, 16. (4) Comes, F. J. Eer. Bunsen-Ges. Phys. Chem. 1990, 94, 1268. (5) Hall, G. E.; Houston, P. L. Annu. Rev. Phys. Chem. 1989,40, 375. Houston, P. L. J. Phys. Chem. 1987, 91, 5388. (6) Simons, J. P. J . Phys. Chem. 1987, 91, 5378. (7) Engel, V.; Staemmler, V.; Vander Wal, R. L.; Crim, F. F.; Sension, R. J.; Hudson, B.; Andresen, P.; Hennig, S.; Weide, K.;Schinke, R. J . Phys. Chem. 1992, 96, 3201 and references therein. (8) Andersen, P.; Ondrey, G. S.; Titze, B.; Rothe, E. W. J. Chem. Phys. 1984,80,2548. Hausler, D.; Andresen, P.; Schinke, R. J . Chem. Phys. 1987, 87, 3949. (9) Grunewald, A. U.;Gericke, K. H.; Comes, F. J. Chem. Phys. Lett. 1987, 133, 501. Mikulecky, K.;Gericke, K.H.; Comes, F. J. Chem. Phys. Lett. 1992, 182, 290. (10) David, D.; Strugano, A.; Bar, I.; Rosenwaks,S.J . Chem. Phys. 1993, 98, 409. (11) David, D.; Bar, I.; Rosenwaks, S.J . Chem. Phys. 1993, 99, 4218. (12) Engel, V.;Schinke, R.; Staemmler, V.J. Chem. Phys. 1988,88,128. (13) David, D.; Strugano, A.; Bar, I.; Rosenwaks, S . Appl. Spectrosc. 1992, 46, 1149. (14) Oka, T. In Molecules in the Galactic Enuironment; Gordon, M. A,, Snyder, L. A., Eds.; Wiley-Interscience: New York, 1973; p 257. (15) Dieke, G. H.; Crosswhite, H. M. J . Quantum. Spectrosc. Radiar. Transfer 1962, 2, 97. (16) Greene, C. H.; Zare, R. N. J. Chem. Phys. 1983, 78, 6741. (17) Dixon, R. N. J . Chem. Phys. 1986, 85, 1866. (18) Hall, G. E.; Sivakumar, N.; Chawla, D.; Houston, P. L.; Burak, I. J . Chem. Phys. 1988,88, 3682. Hall, G. E.; Sivakumar, N.; Houston, P. L.; Burak, I. Phys. Rev. Lett. 1986, 56, 1671. (19) Comes, F. J.; Gericke, K. H.;Grunewald,A. U.; Klee,S. Eer. EunsenGes. Phys. Chem. 1988,92,273. Grunewald, A. U.;Gericke, K. H.; Comes, F. J. J . Chem. Phys. 1987,87, 5709. Gericke, K. H.; Klee, S.;Comes, F.J.; Dixon, R. N. J. Chem. Phys. 1986, 85, 4463. (20) Nagata, T.; Kondow, T.; Kuchitsu, K.;Loge, G. W.; Zare, R. N. Mol. Phys. 1983, 50, 49. (21) Jonah, C. J . Chem. Phys. 1971, 55, 1915. (22) Busch, G. E.; Wilson, K. R. J . Chem. Phys. 1972, 56, 3638. (23) Loock, H. P.;Cao, J.; Qian, C. X.W. Chem. Phys. Lett. 1993,206, 422.
