J. Phys. Chem. 1995,99, 13670-13679
13670
State-to-State Photodissociation of Oriented HF-HCl Complexes: Isotopic and Isomeric Effects L. Oudejans and R. E. Miller* Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599 Received: March 7, 1995; In Final Form: May 15, 1 9 9 9
Photofragment angular distributions have been obtained for the vibrational predissociation of both isomers of the HF-HC1 binary complex. State-selective excitation of the parent complex is carried out using a highresolution F-center laser. To obtain a complete assignment of the data in terms of final state distributions of the fragments, it was necessary to both orient the parent molecules in a dc electric field and state selectively detect the HF fragments using a second probe laser. The dissociation energy of the HF-HC1 isomer is determined to be 642 dz 2 cm-I. Dissociation of this complex leads to the production of a v = 1 HC1 fragment. The final state distributions for the two isotopomers (35CY37C1)are quite different even though the energetics associated with these two systems are essentially the same. The analysis of the HC1-HF distributions, although more qualitative, suggests that the V-V channel is dynamically blocked by the heavy chlorine atom, resulting in the production of v = 0 HC1.
Introduction There has been a remarkable growth in the number of laboratories carrying out spectroscopic studies of weakly bound complexes at every accessible wavelength.'-8 The high resolution that is now available in many of these experiments allows for characterization of the individual quantum states of these complexes. Data of this type has been shown to be extremely sensitive to the detailed intermolecular potential energy surface in the region of the attractive well.9310The theoretical methods for handling the intermolecular vibrational dynamics needed to extract such information from the spectra have also undergone remarkable advances in recent The real power of this approach can be appreciated if one considers that all of the traditional experimentalmethods for determining intermolecular forces, including crossed molecular beam studies,I4 are unable to handle systems where the interaction depends on more than one or two intermolecular coordinates. In contrast, the spectroscopic methods are now being used to study systems with potentials that depend on many degrees of f r e e d ~ m . ' ~This is possible due to the fact that different quantum states of the complex are sensitive to different regions of the potential energy surface, thus allowing one to systematically explore the surface. These complexes are also of great interest owing to the fact that they can be easily dissociated due to the weakness of the intermolecular bond. Although the energy required for dissociation is low, the rate of dissociation is small due to the weak coupling between the initially excited state of the complex and the dissociative coordinate. For example, near-infrared excitation of an intramolecular vibration associated with one of the monomer units in the complex is often sufficient to dissociate the complexI6 on time scales ranging from nanoseconds to milliseconds. The associated line broadening in the infrared spectrum has been used to quantify the rate of this dissociation process in many systems, with the result being that some predictability is now p o ~ s i b l e .The ~ ~ more ~ ~ difficult challenge of characterizing the final states of the fragments is less well advanced, although progress is also being made in this direction as ell.'^-^^ It is already clear from this work that new insights into the nature of the dissociation process can be obtained with
* To whom correspondence should be addressed. @
Abstract published in Advance ACS Absrracfs, August 15, 1995.
0022-3654/95/2099-13670$09.00/0
the additional information provided by these state-to-state studies. It is this aspect of the work that forms the focus of the present study. In a number of recent publications,'8-21.28we have demonstrated the utility of a molecular beam method, based upon the measurement of photofragment angular distributions, that can provide final state distributions resulting from the vibrational predissociation of weakly bound complexes. For example, in systems where the total vibrationalhotational density of final states is very low, as in the HF dimer,18.28.29 individual states can be resolved in the angular distributions. In these cases, dissociation e n e r g i e ~ , l ~ . intermolecular ~'.~~ scalar correlations,18,28and vector correlation^^^^^^ can be determined. Recently, we have extended this method by including a second probe laser to determine the final state distributions spectros c ~ p i c a l l y .In~ ~our most recent work in the area we have shown the utility of orienting the parent complex before it is dissociated.31,32 In the present study we make use of these methods to study the dissociation dynamics of the HF-HCl complex. This complex has been studied previously by both s p e c t r o ~ c o p i c ~ ~ ~ ~ ~ and t h e ~ r e t i c a l methods. ~ ~ . ~ ~ Our interest in this system stems from two facts. First, this complex exists in two isomeric forms that can be readily formed in a free jet expansion, namely, those shown in Figure 1. As a result, we have the opportunity to explore the influence of the initial state geometry on the dissociation dynamics. Second, in the original infrared spectroscopic study of this complex by Fraser and Pine,34 they observed a rather remarkable isotope effect that has yet to be understood. In particular, they found that the vibrational predissociationrate for this complex is strongly dependent upon the 35CY37C1 isotopic form. Our initial hope was to obtain new insights into this effect by characterizing the final state distribution of the fragments resulting from this vibrational predissociation process. Although the experiments reported here do answer some fundamental questions about the nature of the dissociation process in these systems, the fundamental cause of the effect remains unclear. As we shall show, the use of the orientation method is crucial to the successful characterization of the system. The results reported here also provide an accurate value for the dissociation energy of the HF-HCl complex. 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 37, 1995 13671
Photodissociation of Oriented HF-HCl Complexes HF - HCl
+
HC1- HF
HC'fragment
50 40
Figure 1. Structures for the two isomers of HF-HCl. Probe Laser Pump Laser
I
30 20 10 n
"0
d
2
4
6
8
10
12 14 Laboratory Angle ( 0 )
16
18
20
Figure 3. An angular distribution for the Q&(O) transition of the HFH 3 T l complex. The two somewhat ill-defined features are assigned to the HCl (heavy) fragment, appearing at small angles, and the HF (light) fragment, which is scattered to larger angles. Molecular Beam Figure 2. A schematic diagram of the experimental apparatus used in the present study. The photolysis laser excites the complexes on the axis of rotation, and the probe laser is scanned through the transitions associated with the fragments. The electrodes are positioned so that a large dc electric field can be applied- to the photolysis region in order to orient the parent complexes.
Experimental Section Figure 2 shows a schematic diagram of the experimental apparatus. Although many of the \details are given elsehere,'^,^*-^' a brief'overview is presented here. The molecular beam is formed by expanding the gases of interest through a nozzle and collimating the expansion with two skimmers. An F-center laser intersects the molecular beam on the axis of rotation of the molecular beam source, where a spherical multipass cell is used to obtain approximately 60 passes of the laser with the molecular beam. The laser is tuned into resonance with one of the ro-vibrational transitions of the parent complex and locked to a 150 MHz evacuated confocal etalon. A photofragment angular distribution is then obtained by varying the angle between the molecular beam and the line joining the photolysis volume to the bolometer detector in 0.25" incredents. As discussed in detail e l ~ e w h e r e the , ~ ~angular ~ ~ ~ distributions often have considerable structure which reflects the final state distribution of the fragments. The second F-center laser shown in Figure 2 is used to state selectively probe the fragments produced by the pump laser.30931 With the bolometer set at a nonzero angle, the probe laser can only interact with HF monomers that are produced as a result of photodissociation of the complex. The HC1 fragment cannot be probed in this way since the associated vibrational frequency lies outside the tuning range of our F-center laser. Since the photofragment signal is due to the combined translational, internal, and adsorption energies of the fragment molecules reaching the detector, vibrational excitation of the fragments by the probe laser will result in an observable bolometer signal. By tuning the probe laser through the various ro-vibrational transitions of the HF monomer, it is possible to determine the relative state populations of these fragments. The electrodes shown in Figure 2 were used to apply a dc electric field to the parent molecules. They are highly polished and anodized aluminum discs, 0.75 cm in diameter and separated by 0.5 cm. The photolysis laser is confined to an 0.2 cm spot midway between the electrodes. Since the HF-HC1 and HClHF complexes have moderately large dipole moments, they can be oriented along the electric field d i r e ~ t i o n . ~ Assuming ~-~~
that the complex dissociates along the intermolecular axis, this orientation ensures that the HF and HCl fragments scatter to opposite sidks of the apparatus, allowing us to detect them separately. A detailed account of the use of dc electric fields to orient molecules in photodissociation experiments is given elsewhere.32 As we will see in a later section, this method was extremely helpful in obtaining a complete assignment of the final state distributions. In the present study the molecular beam was formed by expanding a mixture of 1% HF and 5% HCl in helium through a 40 pm diameter nozzle, from a source pressure of 550 P a . Since the recoil angle of the fragments depends upon the stream velocity of the molecular beam, it was necessary to use Doppler spectroscopy to measure this for the conditions used in each experiment. The stream velocity determined for the present experiments is 1620 d s . Experimental Results As the study of Fraser and Pine34has shown, there are two isomers of the HF-HCl binary complex, one corresponding to each molecule acting as either a proton donor or acceptor. Combining this with the fact that there are two isotopic forms of each complex (35ClP7C1)gives us a total of four species to study. Since the spectroscopy of all of these have been discussed in detail in the previous work,34 we can use this directly to guide our present dynamical studies. HF-H35Cl: Ka = 0 0 Subband. As one would expect from isotopic abundance, the 35Cl species gives rise to the strongest transitions in the spectrum. Since the isomer considered here has the HC1 acting as the proton donor, the H-F stretch is "free", and thus the line width is quite narrow. It is interesting to note that for the Ka = 0 0 subband considered here the predissociation line width is 38 MHz in the 35Cl complex and 29 MHz for the 37Clcomplex.34 The preliminary experiments involved recording the angular distribution resulting from excitation of the R(0) transition in the& = 0 0 subband, located near 3940.6 cm-'. From our previous studies on the HF dimer,29we know that the distributions obtained from this transition, excited with the laser polarization perpendicular to the molecular beam, give the best resolution of the individual final state channels owing to the alignment that is imposed by the laser. The resulting angular distribution for €-IF-H~~C~ is shown in Figure 3. Unfortunately, the lack of structure in this angular distribution does not allow us to obtain an assignment, since there are many different combinations of final state channels that could give rise to such a distribution. The
-
-
-
Oudejans and Miller
13672 J. Phys. Chem., Vol. 99, No. 37, 1995 180
w=l0.6
-
‘HFH35C1
160 140 -
Figure 4. A schematic showing the orientation of HF-HCl in an electric field. The oval-shaped distribution shows a slice through the square of the M = 0 pendular state wave function, corresponding to
120
.
100 -
the orientation probability distribution. difficulty is compounded by the fact that we do not know the dissociation energy of this complex, so that the identity of the final states corresponding to this range of recoil energies is not known. The lack of structure in this angular distribution arises from two factors. First, there is the fact that the two fragments have different masses and thus recoil to different angles in the laboratory frame. Since the bolometer does not discriminate between the two fragments, the two distributions overlap, increasing the congestion in the angular distribution. In fact, the two ill-defined features that are observed in this angular distribution are due to the HCl (heavy) fragment, which appears at small angles, and the HF (light) fragment, which is scattered to larger angles. Second, in comparison with the HF dimer, there are many more channels in the present case, partially due to the fact that the HC1 fragment has a rotational constant which is half that of the HF. In addition, the two fragments are distinguishable in the present case, effectively doubling the number of available states. The best that one could hope for from an angular distribution such as this is the average kinetic energy release. Since this value is quite often small for van der Waals molecule dissociations, it is generally not a very informative quantity. In our previous study of the N2-HF ~ o m p l e x , ~ we’ showed that the problems associated with the overlapping of the two angular distributions could be solved by orienting the complex prior to dissociation, so that the two fragments would recoil in opposite directions in the laboratory frame. This is easily accomplished by applying a dc electric field to the photolysis volume using the electrodes shown in Figure 2. For the present experiments we used an electric field of 30 kV/cm. For the HF-H3Tl complex, which has a rotational constant B = 0.1 14 cm-’ 34 and a dipole moment p = 2.4095 D,33this corresponds to an w = O.O1679pE/B = 10.6. As shown elsewhere,32this value of w is large enough to ensure that all the molecules are sufficiently oriented so that none of the fragments are expected to scatter in a direction opposite to the one they are pointing, assuming the recoil is axial. This is illustrated by the twodimensional slice through the probability distribution shown in Figure 4 for an M = 0 pendular state calculated at w = 10.6. Given the direction of the dipole moment in +HF-HCl-, we expect to observe the HC1 fragment on the side of the apparatus corresponding to the positive electrode and the HF fragment on the negative electrode side. In practice, after first checking for symmetry, all of the angular distributions were recorded on one side by simply reversing the polarity on the electrodes. The f i s t step in these measurements is to identify the spectral features associated with the pendular spectrum. Since this is a Ka = 0 0 subband and the laser polarization is set to be parallel to the dc electric field direction, we expect to observe the pendular type transitions near the vibrational origin.32 Indeed, upon application of the field we observed a single transition in this region of the spectrum, which showed no signs
-
y%
I
I 8
it
-
3
s3
HCI fragments
80 -
6040
-
HF fragments
i\
Laboratory Angle ( ’)
Figure 5. Angular distributions resulting from photodissociation of
oriented HF-H3sC1. The HCl fragments are scattered toward the positive electrode and the HF toward the negative electrode, in agreement with our expectations from Figure 4. of splitting up to fields as high as 60 kV/cm. As we have shown the splitting of this pendular state feature is caused by changes in the rotational constant and the dipole moment upon vibrational excitation. Since we are exciting a “free” HF stretch in this case, the corresponding changes are small and the pendular spectrum does not split. In principle, this is a problem since the best orientation is achieved by selectively pumping only the M = 0 excited state. However, by positioning the laser to the low-frequency side of the transition, we preferentially pump the low (mostly zero) M states. In any case, the w value used here is sufficiently high to ensure that the molecules are strongly oriented, as is evident from the angular distributions presented below. It is interesting to note that since the orienting field focuses the molecules into the plane of the detector, the signals with the field applied were approximately a factor of 4 larger than for the RRo(0)transition in zero field. Figure 5 shows angular distributions obtained using the pendular state method for both the positive and negative electrode sides of the apparatus. Careful inspection of the results reveals that the distribution on the negative electrode side of the apparatus extends to larger angles compared with the positive side. This is consistent with the fact that on the negative electrode side it is the lighter HF fragment that is detected. What is most encouraging is the fact that there is considerable structure on the HF side of the distribution now that the overlapping contribution from the HC1 has been eliminated. This will clearly be of great help in trying to assign the individual final states. However, before going on to discuss this aspect in detail, it is interesting to consider the qualitative features of this distribution. Perhaps the most striking feature is the large difference in the signal intensity between the two sides. Indeed, the signals on the HC1 side are a factor of 5.2 larger than on the HF side. There are a number of things that could contribute to this difference. First, since the HC1 fragment recoils to a smaller angle in the laboratory frame than does the HF fragment, there is a Jacobian factor that favors the HC1 signal. Second, the mass of the HC1 fragment is a factor of 35/20 = 1.75 larger than HF, which means that, since all the molecules are traveling at the same stream velocity, the translational energy of the HC1 fragment is also larger. The bolometer being an energy detector will be sensitive to this difference. On the other hand, the
J. Phys. Chem., Vol. 99, No. 37, 1995 13673
Photodissociation of Oriented HF-HCl Complexes adsorption energy of the HCl to a layer of HCl on the surface of the bolometer is expected to be lower than that of HF on HF, based on the relative dissociation energies for the two dimers, namely, 1062 and 431 cm-’ for HF and HC1 dimers, r e s p e c t i ~ e l y .This ~ ~ ~will ~ ~ tend to offset the other two effects. As a result, these factors taken together can account for a difference of less than a factor of 2. We are therefore forced to conclude that the HCl fragment has considerably more internal energy than the HF. The most likely way for this to occur is if the predominant dissociation channel leaves the HC1 fragment in u = 1, which is an open channel for this complex. When this factor is included in our energy calculation, the relative intensities of the two angular distributions are explained quantitatively. From this qualitative examination of the angular distributions we can already make the tentative assignment that the dominant mechanism for dissociation involves vibrational to vibrational energy transfer from the H-F stretch in the complex to the H-Cl stretch in the fragments. It is clear from the above results that the pendular state method allows us to uncover structure in the HF angular distribution that was obscured in the zero-field data. Nevertheless, it is important to rule out the possibility that the features in the small angle region of the HF angular distribution are not due to “backscattered” HC1. Fortunately, this possibility can easily be discounted by the fact that the peaks appearing in the HF angular distribution do not match those on the HCl side. Therefore, as expected from the pendular state wave functions, we see no evidence of backscattered HCl. Further progress can now only be made by carefully examining the structure seen in the HF fragment distribution. Given the qualitative assignment discussed above, we consider here only the open channels associated with the HCl u = 1 fragment. This assumption will be shown to be valid shortly. The structure observed in the HF distribution is clearly associated with different rotational states of the fragments that have different translational energies. Each fragment channel is characterized by the two vibrational quantum numbers (UHF,VHC~), which we take to be (0, 1) and two rotational quantum numbers UHFJ’HCI). The difficult part of the assignment process comes from the fact that we do not accurately know the dissociation energy of the complex. The recoil energy of the fragments is dependent upon the dissociation energy according to
so that any final state can be “tuned” to any angle by simply changing the dissociation energy. As a result, one must rely on the pattem of peaks in the angular distribution to determine the dissociation energy. If there is only one dissociation energy that gives rise to the experimentally observed pattem of energy levels, then the assignment is unique. Unfortunately, we find that there are three possible dissociation energies that result in an energy level pattem that can explain the experimental data, namely, 338,642, and 706 cm-I. Any one of these values gives an equally good fit to the angular distribution, using a Monte Carlo program to average over the instrumental parameters discussed p r e v i o u ~ l y . ~ For ~ * *each ~ value of DO,however, the final state distributions are different. It is interesting at this point to consider what is currently known about the dissociation energy of the HF-HCl complex. Augsburger and D y k ~ t r ahave ~ ~ calculated the dissociation energy for both isomers of HF-HCl. For the present case, where the HC1 acts as the proton donor, they obtain a value of 415 cm-I. It is informative to note that for the HC1 dimer. where the bonding is similar to the present case, Augsburger and D ~ k s t r aobtain ~ ~ a value of 337 cm-l. in comDarison with
Photofragment Energies *HFWJCL
3400
K=l 3300
-
{?:I (3.41
K=O -
h
7E
2 x
g w
3200
a’ =. 31 00
3000
Figure 6. An energy level diagram for the vibrational predissociation of HF-HC1 corresponding to DO= 642 cm-l. The K = 0 and K = 1
labels indicate states of the complex, while the fragment states associated with HC1 v = 1 are shown on the right along with their rotational quantum number assignments.
I
the experimentally determined value of 431 c ~ - ’ . ~ O The fact that they underestimated the value for the HC1 dimer suggests that the lowest of the three values we obtained above for HFHC1 is probably not the correct one. Indeed, if we scale the theoretical value for HF-HC1 by the ratio of the experimental to the calculated values for HCl dimer, we obtain an estimate for DOof HF-HCl of 530 cm-I. This is still considerably less than the other two experimental values and obviously cannot be used to quantitatively estimate the dissociation energy of this complex. To differentiate between the three experimental possibilities for DO, we need a method that can label the peaks in the angular distribution with quantum numbers associated with at least one fragment. Since the three dissociation energies give rise to different quantum number assignments to these peaks, such a determination would allow us to choose the correct value. This was done by using the second laser shown in Figure 2 to identify the HF rotational states associated with the observed peaks in the angular distribution. With the apparatus set to detect the peak at 6”, the second laser was scanned through the various HF ro-vibrational transitions. At this angle signals were only observed on the P(3) transition. This immediately tells us that this peak is associated with a channel with UHF = 3, ~ H C I ) . When the apparatus was moved to the peak located at 9.5”, we also observed only j H F = 3. However, when the apparatus was positioned between these two peaks, signals were also observed corresponding tojHF = 2. Finally, the peak at 12’ only showed signals on the P(3) transition. The only value for the dissociation energy that-is consistent with this j H F distribution is DO= 642 cm-I. Figure 6 shows an energy level diagram applicable to this dissociation energy. The two states labeled K = 0 and K = 1 correspond to the excited states of the complex, shifted in energy by the dissociation energy determined above. The fragment channels shown on the right side of the figure correspond to only the VHC~= 1 channels. The two-laser experiment very
13674 I. Phys. Chem., Vol. 99, No. 37, I995
Oudejans and Miller 0.30
36
180
Experiment
- Monte Carlo Fit
32
9 9
,x
-z
i0.30
Experiment Monte Carlo Fit
0.25
0.25
28 0.20
24
0.20
P
.e
g
r
i3
s 0.15 2
.LI
20
0.15
16
8
12
'
8
0.10
3
h
0.05
'
80 60
0.10
h
40 0.05
20
4 0
O
2
2
25
2 2
12
^
20
E
16
s
2
12
'I
'
40
h
4
'
HC1 fragments
30
I
8
p
4
2
3 .:
0,
= a-_ *... -I
-6 5 &
20
10
Figure 11. Expanded view of the HF angular distribution for HFH3'Cl. The solid line through the points is the result of a Monte Carlo fit to the data yielding the probabilities shown by the vertical bars.
A
0 -20
The quantum number assignments for each channel are indicated under the vertical bars.
-
r
t
HF fragments
Laboratory Angle ( ")
the calculated intermolecular bending (146 cm-') and stretching (87 cm-I) frequencies for the complex, reported by Augsburger and D y k ~ t r awe , ~ ~estimate that the dissociation energy should become approximately 1 cm-' larger in going from 35Cl to 37Cl. The estimated change in the stretching mode frequency is based upon a pseudodiatomic model, and the change in the bending frequency was based upon the difference in the corresponding monomer rotational constants. Given that the monomer states are also only weakly effected by this isotopic change,35we conclude that the effect we are seeing here is not due to changes in the energetics of the dissociation process. Indeed, the energy level diagram for the 37Cl species is indistinguishable from the one for the 35Clcomplex on the scale shown in Figure 6. In fact, the energy shifts associated with the parent complexes and the fragments essentially cancel, making the energetics the same for most channels within a fraction of a wavenumber. This is illustrated by the recoil energies given in Table 1 for both isotopomers. On the basis of this, we expect that recoil angles for the various channels will be same as for the 35Clcomplex. A complete analyses of the 37Cldata reveals that the various channels do indeed appear at the same angles in the angular distribution. Thus, the differences we see in the angular distributions are entirely due to differences in the state-to-state probabilities of these channels. Figure 11 shows an expanded view of the HF side of the angular distribution, again with a fit and the corresponding probabilities. The best fit was obtained with a dissociation energy of 644 cm-I for the 37Clcomplex. The 2 cm-' difference from the value given for the 35Clcomplex is just at the lower edge of the error bar. Nevertheless, a significantly better fit was obtained with this slightly larger number than when using the value determined for the 35Cl species. The fact that the experimental value is slightly larger for 37 clearly makes sense in terms of the zero point energy arguments. Once again, the HC1 side of the angular distribution was accurately reproduced by the same set of probabilities as determined from fitting the HF side. These probabilities are summarized in Table 1. It is important to point out again that the channel appearing at 2" (4,O)has a large uncertainty due to interference from the primary molecular beam. As a result, the large probability shown for this channel must be viewed with caution. H35Cl-HF: Ka = 0 0 Subband. We now turn our attention to the other isomer of this complex, namely, the one with the HF acting as the proton donor. Once again the spectroscopic road map has already been drawn for this complex
ti 1
-10
0
10
20
Laboratory Angle ( O)
Figure 12. Angular distributions resulting from excitation of the H3%ZI-HF complex. Note that the HC1 is now scattered to negative angles and the HF to positive angles.
by Fraser and Pine.34 In this case, since the H-F bond is essentially directly along the A axis of the complex, only the Ka = 0 0 subband is observed in the spectrum. This isconvenient for the present purposes since the pendular state spectroscopy of such a band is straightforward and the same as discussed above. The one difference is that we are now exciting a vibrational coordinate that is directly involved in hydrogen bonding. As a result, the rotational constant and dipole moment change by a larger amount, and the pendular state spectrum can be resolved. In this case we pump the pure M = 0 state. The conditions used for these experiments were identical to the ones discussed above for the other isomer. Figure 12 shows the angular distributions obtained for this isomer. As expected, the HCl and HF fragments now appear on opposite sides of the apparatus compared with the earlier results. This is a result of the fact that the polarity of the complex is reversed, namely, +HCl-HF-. Once again, there is much more structure evident on the HF side compared with HC1. One thing that is immediately obvious from these distributions is that the ratio of the maximum signal on the HCl side to that on the HF side is somewhat smaller, namely, 3.0 instead of 5.2 for HF-H35Cl. The implication is that the HF fragment produced from vibrational predissociation of the HC1HF isomer has relatively more internal energy in this case, compared with the corresponding fragment from the other isomer. It is tempting to explain this difference purely on the basis that the HF is now in the proton donor position. In our earlier work on the HF dimer,**we showed that the preferred channels are those where one HF is highly rotational excited while the other is not. This was interpreted as meaning that an impulsive dissociation of the complex leads to preferential rotational excitation of the proton donor, based simply on the torques that are acting. Therefore, since the HF is in the proton donor position in the HCl-HF complex we might expect to see greater rotational excitation of the HF fragment in this case. The difficulty with this argument is that there is simply not enough internal energy available to account for such a large effect, assuming that the HCl fragment is still produced in the v = 1 state. Indeed, in earlier sections of this paper we have already shown that many of the important channels in the HF-HC1 isomer already have three or four quanta of HF rotational energy, which accounts for most of the available intemal energy
-
Photodissociation of Oriented HF-HC1 Complexes remaining after excitation of the HC1 vibration. As a result, the only way we can account for this difference is to assume that the dissociation of the HCl-HF isomer leads to the production of at least some 2, = 0 HC1, which would then make enough energy available in the HF fragment to account for the observed differences. Unfortunately, to date we have been unsuccessful in observing the probe laser signals for this isomer, which is really essential to make a complete rotational assignment. Work is underway to improve the signal-to-noise ratio such that these transitions can be observed and the detailed rotational distributions determined. Although the lack of the pump-probe results prevents us from doing a complete assignment, similar to the one reported above for HF-HC1, we can proceed to make a tentative assignment of the data for this isomer. Even though we are unable to determine the dissociation energy of HC1-HF from these measurements, we can look at the relative intensities of the HF and HCl angular distributions to obtain some very interesting insights into the dissociation process. Without the spectroscopic final state probe, we find that there are several dissociation energies that give a good fit to the experimental data. Nevertheless, in every case where only the u = 1 channels of the HC1 were included in the fit, the intensity of the HCl angular distribution was always too large in comparison with that of HF. To better appreciate the magnitude of the overshoot, we note that to bring these calculations into agreement with the experiment, the sticking energy of the HC1 would have to be reduced to approximately zero. From this we conclude that the predominant dissociation channel for this isomer involves the production of ground vibrational state HC1. In this experiment we effectively use the bolometer as a calorimeter to determine the relative energies of the fragments, providing at least quantitative information on the intemal states of the fragments. Again, this is made possible only by orienting the parent complexes.
Discussion From the recoil energies and probabilities listed in Table 1, we can calculate the average kinetic energy release characteristic of the various distributions associated with HF-HC1. For the 35Clcomplex, dissociation from the Ka = 0 level results in an average kinetic energy of 112 cm-I, compared with 58.4 cm-’ for the same state of the 37Cl complex. This is evident from the fact that the former angular distribution shown in Figure 7 extends to larger angles than the corresponding one for the 37Clcomplex shown in Figure 11. It is important to remember that the relative energies of the various parent complex and fragment states are the same within fractions of a wavenumber for these two complexes. As a result, this difference is clearly representative of a real dynamical effect, which is also manifested in the predissociation rates, as observed by Pine and F r a ~ e r .What ~ ~ is puzzling about this result is that the larger average kinetic energy is associated with the complex that has the broader line width and thus the shorter line width. Indeed, the 35Cland 37Clcomplexes have line widths in this vibrational band of 38 and 29 MHz, r e ~ p e c t i v e l y .This ~ ~ is at odds with energy gap arguments43which suggest that the lifetime should become longer as the average kinetic energy release increases. Exactly the opposite trend is seen in going to the K = 1 state of the 35Clcomplex. In this case, the average kinetic energy is only 47.9 cm-’, and the line width increases to 82 MHz. Unfortunately, the signals associated with the very broad (246 MHz) transitions of the 37Clcomplex in Ka = 1 are very weak, preventing us from obtaining reasonable angular distributions. It is interesting to note that all of the line widths associated
J. Phys. Chem., Vol. 99, No. 37, 1995 13677 with the HF-HCl complex are 6-8 times larger than those of the free H-F stretch of the HF presumably due to the fact that in the present case we have an open V-V channel that accelerates the dissociation processes. For the H3T1-HF isomer Pine and F r a ~ e observed r~~ a line width of 142 MHz, which is again anomalous from several points of view. First, in comparison with the HF dimer where the ratio of the lifetimes associated with the free and the hydrogen-bonded H-F stretches is 22, the differences here are much smaller for the present case. Indeed, some of the free H-F stretch transitions ( K = 1) associated with the 37Clspecies are actually broader than those associated with the hydrogenbonded mode of the 35Cl complex. We take this unusual behavior as evidence that the two isomers undergo quite different dynamics. As noted above, the lifetime of the “free” HF stretching vibration of HF-HCl is 6-8 times shorter than that of the corresponding mode in the HF dimer. Although this V-V channel is also energetically open in the HC1-HF case, the lifetime of this complex is approximately twice that of the corresponding hydrogen-bonded mode of the HF dimer. The implication is that the V-V channel is dynamically hindered in some way so that it is less important in controlling the dissociation rate. In fact, since this would necessitate dissociation via V-R channels, the smaller rotational constant of the HC1 (in comparison with HF) would make it more difficult to dispose of the available energy (which is to say that higher J states have to be accessed), slowing down the dissociation rate. This is all in good agreement with the present observations that show that the HF fragment has relatively more intemal energy upon dissociation of HC1-HF, when compared with HF-HCl. This dramatic dynamical difference can also be rationalized on the basis of the structures for these two complexes, shown in Figure 1. In the case of the HF-HCl complex, the receptor H-Cl vibration is directly coupled to the intermolecular bond so that as the molecule dissociates the necessary forces will be present to excite the H-Cl vibration. Even though the H-F vibration is rather decoupled, we know that dissociation can only occur as the energy finds is way into the intermolecular bond and thus is able to communicate with the H-Cl vibration. The situation is very different for the case of the HC1-HF complex. Since all of the motions associated with the V-V mechanism involve primarily the hydrogen atoms, it is apparent that the hydrogen of H-Cl will know little of the dissociation process that is going on. The heavy chlorine atom effectively blocks all communication between the excited H-F vibration and that of the H-C1. Although wide amplitude bending motions could increase the direct coupling between the hydrogens, the hydrogen bonding will clearly strongly disfavor having the two hydrogens close together. In any case, all of the data discussed above suggest that the V-V channel is at least partially blocked in the HCl-HF complex, which indicates that the coupling between the hydrogen atoms motions is indeed weak. This is a very beautiful example of a system in which a heavy atom acts as a dynamical block to the flow of vibrational en erg^.^^-^^ Namely, an open channel, which we know from the other isomer can accelerate the dissociation process, is dynamically hindered in this isomer, forcing the system to dissociate via a different pathway. Consider for a moment the rotational distribution associated with the dissociation of the HF-HCl complex. As noted above, most of the available energy in this case is deposited directly into the vibrational degree of freedom of the HC1. As a result, in comparison with HF dimer, relatively little rotational energy appears in the fragments. Figure 13 shows a comparison between the observed angular distributions for the HF-H35C1
13678 J. Phys. Chem., Vol. 99, No. 37, 1995 180 160
5 I
Oudejans and Miller
----_ PST. only
one rotor and the fact that the kinetic energy, and thus the translational degeneracy, is also very small. Nevertheless, the experiments clearly show the presence of this channel, indicative of a strong nonstatistical preference for this channel. This is perhaps not unexpected given that this channel is only nonresonant by 3 cm-I and could therefore be strongly enhanced by the resonance.
v ~ -1 a PST, all channels
1
..
9 .
,401 120
9
.
Summary
-30
-20
-1 0 0 10 Laboratory Angle (O)
20
In the above sections we have presented a wealth of new data on the vibrational predissociation of HCl-HF and HFHCl. In the latter case, the dissociation energy is accurately measured (DO= 642 cm-I), and the final state distributions are obtained for two different initial states of the 35Clcomplex ( K = 0 and K = 1) and for the K = 0 state of the 37Clspecies. In these cases, the intermolecular internal state correlations are obtained, showing that the HC1 fragment is selectively produced in the v = 1 vibrational level. Large differences are observed in the final state distributions for the two isotopomers. Although the exact cause of this difference, as well as that in the associated predissociation lifetimes, remains somewhat unclear, the implication is that the intermolecular V-V coupling is in some way strongly dependent upon the mass of the chlorine. Nevertheless, the fact that the energetics is the same for both isotopomers means that this cannot be understood simply in terms of energy resonances. For the second isomer, the results are still preliminary but do show that the dissociation process proceeds via a very different mechanism which leads to the production of primarily v = 0 HCl. This behavior can be rationalized in view of the very poor coupling between the H-F and H-Cl vibrations in this complex due to the location of the heavy chlorine atom, which effectively blocks the flow of vibrational en erg^?^-^^ Standing between the two hydrogen atoms, the chlorine effectively isolates the hydrogen atom motions, in much the same way the oxygen atom does in the water molecule. As a result, the system is forced to dissociate via the much less desirable V-R pathway, resulting in the anomalously long lifetime for a vibration that is directly coupled to the dissociation coordinate.
30
Figure 13. Comparison between the experiment angular distributions for HF-H3Tl and those obtained from phase space theory calculations. The dashed line is for a PST calculation where all open channels are included (both u = 0 and u = 1 HC1) while the solid line is a PST calculation only including u = 1 HC1. 90
,,
.
- PST calculations .+Experiment
80 -
40
t
-0
A
2
4
6 8 10 12 1 4 16 Laboratory Angle ( 9
Figure 14. Comparisons between the PST calculations and the experimental angular distributions for the HF fragments resulting from vibrational predissociation of HF-H3T1 and HF-H3’C1.
complex and those obtained from statistical phase space theory (PST) c a l ~ u l a t i o n s , ~ in~one ~ ~ ’case including all open channels and in the other including only the states associated with v = 1 HCl. The former calculation clearly gives results that are in qualitative disagreement with experiment, indicating that the selection of the v = 1 level is indicative of a nonstatistical dynamical preference. On the other hand, the phase space calculation based only on the v = 1 channels is in nearly quantitative agreement with experiment. This is shown in more detail for the HF distributions associated with both HF-H35C1 and HF-H37Cl in Figure 14. The agreement is clearly much better for the 35Clspecies than for the 37Clcomplex. The reason for this is unknown. The PST probabilities resulting from including only the HCl u = 1 states are given in Table 1 for the case of the HF-H35Cl complex. It is worth pointing out, however, that due to the fact that the energies for both isotopic forms of the complex are so similar, the PST results for the HF-H3’C1 complex are essentially the same. What is interesting about the comparisons between experiment and PST calculations shown in Figure 14 is the absence of intensity in the nearly resonant (4,O) channel in the latter. This can be understood in terms of the low rotational degeneracy resulting from all the rotational angular momentum being in
Acknowledgment. This work was supported by the National Science Foundation (CHE-93-18936). We also acknowledge the donors of The Petroleum Research Fund, administered by the ACS, for partial support of this research. References and Notes Klemperer, W. Springer Ser. Chem. Phys. 1978, 3, 398. Klemperer, W. Nature 1993, 362, 698. Levy, D. H. Adu. Chem. Phys. 1981, 47, 323. Miller, R. E. Science 1988, 240, 447. (5) Nesbitt, D. J. Chem. Rev. 1988, 88, 843. (6) Pugliano, N.; Saykally, R. J. Science 1992, 257, 1937. (7) Celii, F. G.; Janda, K. C. Chem. Rev. 1986, 86, 507. (8) Berry, M. T.;Brustein, M. R.; Lester, M. I.; Chakravarty, C.; Clary, D. C. Chem. Phys. Lett. 1991, 178, 301. (9) Leroy, R. J.; Carley, J. S. Adv. Chem. Phys. 1980, 42, 353. (10) Hutson, J. M. J . Chem. Phys. 1988, 89, 4550. (11) Bacic, 2.;Light, J. C. J . Chem. Phys. 1986, 85, 4594. (12) Peet, A. C.; Yang, W. Chem. Phys. Lett. 1988, 153, 98. (13) Clary, D. C.; Dateo, C. E.; Stoecklin, T. J . Chem. Phys. 1990, 93, 7666. (14) Gerber, R. B.; Buch, V.; Buck, U.; Maneke, G . ; Schleusener, J. Phys. Rev. Lett. 1980, 44, 1397. (15) Schmuttenmaer,C. A.; Cohen, R. C.; Saykally, R. J. J . Chem. Phys. 1994, 101, 146. (16) Miller, R. E. J . Phys. Chem. 1986, 90, 3301. (17) Leroy, R. J.; Davies, M. R.; Lam, M. E. J . Phys. Chem. 1991, 95, 2167. (18) Dayton, D. C.; Jucks, K. W.; Miller, R. E. J . Chem. Phys. 1989, 90, 2631. (1) (2) (3) (4)
~
Photodissociation of Oriented HF-HCl Complexes (19) Bohac, E. J.; Marshall, M. D.; Miller, R. E. J . Chem. Phys. 1992, 97, 4890. (20) Bohac, E. J.; Marshall, M. D.; Miller, R. E. J . Chem. Phys. 1992, 97, 4901. (21) Bohac, E. J.; Miller, R. E. J . Chem. Phys. 1993, 98, 2604. (22) Cline, J. I.; Sivakumar, N.; Evard, D. D.; Janda, K. C. Phys. Rev. A: Gen. Phys. 1987, 36, 1944. (23) Butz, K. W.; Catlett, D. L., Jr.; Ewing, G. E.; Krajnovich, D.; Parmenter, C. S. J. Phys. Chem. 1986, 90, 3533. (24) Cline, J. I.; Reid, B. P.; Evard, D. D.; Sivakumar, N.; Halberstadt, N.; Janda, K. C. J . Chem. Phys. 1988, 89, 3535. (25) Burak, I.; Hepbum, J. W.; Sivakumar, N.; Hall, G. E.; Chawla, G.; Houston, P. L. J . Chem. Phys. 1987, 86, 1258. (26) Brouard, M.; Simons, J. P.; Wang, J. X. Faraday Discuss. Chem. Soc. 1991, 63. (27) Serafin, J.; Ni, H.; Valentini, J. J. J . Chem. Phys. 1994, 100, 2385. (28) Bohac, E. J.; Marshall, M. D.; Miller, R. E. J . Chem. Phys. 1992, 96, 668 1. (29) Marshall, M. D.; Bohac, E. J.; Miller, R. E. J . Chem. Phys. 1992, 97, 3307. (30) Bohac, E. J.: Miller. R. E. Phvs. Rev. Lett. 1993. 71. 54. (31) Bemish, R. J.; Bohac, E. J.; W u , M.; Miller, R. E. J . Chem. Phys. 1994, 101, 9457. (32) Wu, M.; Bemish, R. J.; Miller, R. E. J . Chem. Phys. 1994, 101, 9447. (33) Janda, K. C.; Steed, J. M.; Novick, S. E.; Klemperer, W. J . Chem. Phys. 1977, 67, 5162. (34) Fraser, G. T.; Pine, A. S. J. Chem. Phys. 1989, 91, 637.
J. Phys. Chem., Vol. 99, No. 37, 1995 13679 (35) Augspurger, J. D.; Dykstra, C. E. Chem. Phys. Lett. 1992, 189, 303. (36) Latajka, Z.; Scheiner, S. Chem. Phys. 1¶8, 222, 413. (37) Friedrich, B.; Herschbach, D. R. Z. Phys. D:At.,Mol. Clusters 1991, 18, 153. (38) Block, P. A.; Bohac, E. J.; Miller, R. E. Phys. Rev. Lett. 1992, 68, 1303. (39) Rost, J. M.; Griffin, J. C.; Friedrich, B.; Herschbach, D. R. Phys. Rev. Lett. 1992, 68, 1299. (40) Pine, A. S.; Howard, B. J. J . Chem. Phys. 1986, 84, 590. (41) Webb, D. U.; Rao, K. N. J . Mol. Spectrosc. 1968, 28, 121. (42) Rank, D. H.; Rao, B. S.; Wiggens, T. A. J . Mol. Spectrosc. 1965, 17, 122. (43) Ewing, G. E. J . Phys. Chem. 1987, 91, 4662. (44) Huang, Z. S.; Jucks, K. W.; Miller, R. E. J. Chem. Phys. 1986,85, 3338. (45) Pine, A. S.; Fraser, G. T. J . Chem. Phys. 1988, 89, 6636. (46) Vopez, V.; Marcus, R. A. Chem. Phys. Lett. 1982, 93, 232. (47) Lederman, S. M.; Lopez, V.; Voth, G. A.; Marcus, R. A. Chem. Phys. Lett. 1986, 124, 93. (48) Lederman, S. M.; Lopez, V.; Fairen, V.; Voth, G. A.; Marcus, R. A. Chem. Phys. 1989, 139, 171. (49) Uzer, T.; Hynes, J. T. Chem. Phys. 1989, 139, 163. (50) Light, J. C. Faraday Discuss. Chem. Soc. 1967, 44, 14. (51) Light, J. C. J. Chem. Phys. 1964, 40, 3221.
JP950642M
