Laser Transient Absorption Spectroscopy of Bromomethylene

JOURNAL OF MOLECULAR SPECTROSCOPY ARTICLE NO.

188, 68–77 (1998)

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Laser Transient Absorption Spectroscopy of Bromomethylene Andrew J. Marr,* Simon W. North,* ,1 Trevor J. Sears,* ,2 Leah Ruslen,† and Robert W. Field† *Department of Chemistry, Brookhaven National Laboratory, Upton, New York 11973-5000; and †Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received September 18, 1997; in revised form November 10, 1997

We report the observation and assignment of new high-resolution spectra of the A˜ 1 A 9 R X˜ 1 A * transition of bromomethylene, HCBr, obtained by transient laser absorption spectroscopy at near-infrared wavelengths. The 2 10 band of HCBr ( n0 Å 11 957 cm01 ) and the 2 20 band of DCBr ( n0 Å 12 349 cm01 ) have been observed for both naturally occurring isotopes of bromine. The c-type rotational branches of the Ka Å 0 R 1 subbands have been assigned in detail. Other subbands have been partially assigned, but their detailed rotational quantum number analysis has not yet proved possible. Their presence does, however, indicate that the molecule exhibits nonlinear rotational structure in these vibrational levels, in contrast to what was found for the A˜ 1 A 9 (0, 2, 0) level of HCBr ( n0 Å 12 786 cm01 ) [B. C. Chang and T. J. Sears, J. Chem. Phys. 105, 2135–2140 (1996)]. Analysis of the rotational structure in the spectra reported here has required a reassessment of certain rotational assignments of that previous work. We now find that the lower singlet state is isolated; there is no evidence of triplet state perturbations. Rotational constants derived for the ground state of all the naturally occurring isotopomers were used to estimate structural parameters. A barrier to linearity for the A˜ 1 A 9 state, 13 590 cm01 above the zero point level of the ground X˜ 1 A * state, is estimated. q 1998 Academic Press

brational levels of the two surfaces gives rise to vibronic wavefunctions of mixed character and vibronic level shifts. Superimposed on this are near-random energy level shifts caused by spin–orbit interactions between these vibronic levels and the background of vibrational levels associated with the triplet surface. Additional local perturbations are caused by Coriolis and anharmonic (Fermi) interactions. The totality of these assorted mechanisms gives rise to the observed spectral complexity. Spectroscopic studies of halomethylenes can shed light on the relative strengths of the different interstate couplings in CH2 . Except possibly for the case of HCI (7, 8), the lowest electronic state is singlet in character, X˜ 1 A *, and the triplet state, corresponding to X˜ 3 B1 in CH2 , lies at increasingly higher energy the lighter the halogen atom. The observation and analysis of the analogous spectra of several halomethylenes have been reported. For HCCl (9–11) the highresolution spectrum of the A˜ 1 A 9 –X˜ 1 A * (10) origin band at 12 280 cm01 is characteristic of a c-type transition within a bent asymmetric top molecule; the few detected perturbations are very small, an observation atypical of all the known high-resolution spectra of this type of molecule. Excitation to one quantum of bending vibrational energy in the upper state (11) leads to the observation of many more perturbations as the molecule approaches linearity and the Renner– Teller and spin–orbit interactions increase in strength. For bromomethylene, we recently recorded a section of the singlet–singlet spectrum at high resolution for the first time (12). The 2 20 band was measured, which corresponds to excitation from the zero point level of the ground state to the excited state vibrational level with two quanta of bend. Note that the three vibrational modes of bromomethylene,

I. INTRODUCTION

The spectroscopy of methylene, CH2 , has provided a benchmark for advances in experimental and theoretical techniques for more than 30 years. The number of papers in the literature is vast and the reader is referred to comprehensive reference lists in several recent papers (1–4). Despite such efforts, the visible and near-infrared spectrum of methylene, in common with other simple carbenes studied, has complexities that still defy complete analysis. All simple carbenes possess at least three low-lying electronic states and the observed spectral complexities arise from coupling and perturbations between energy levels nominally associated with these different electronic states. For CH2 , the lowest state is a triplet (X˜ 3 B1 ), which lies some 3150 cm01 (5) below the a˜ 1 A1 state. Spectroscopic transitions originating from the latter and terminating at levels nominally associated with the bH 1 B1 state give rise to the visible and near-infrared spectrum first assigned in detail by Herzberg and Johns (6) and much studied subsequently (1, 2). However, at high resolution the observed spectrum is very irregular and contains many strong absorption lines whose rovibronic assignments remain unknown. The origins of the perturbations that produce the observed spectral irregularities have been known, in broad terms, for many years. The a˜ and bH states of the molecule form a Renner–Teller pair (3); they correlate with a degenerate 1 Dg state at linearity and coupling between the bending vi1 Present address: Department of Chemistry, Texas A&M University, College Station, TX 77843. 2 To whom correspondence should be addressed.

68 0022-2852/98 $25.00 Copyright q 1998 by Academic Press All rights of reproduction in any form reserved.

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£1 , £2 , and £3 , denote the C–H stretch, bend, and C–Br stretch, respectively. Here, £2 corresponds to the vibrational quantum number in the bent molecule limit. We identified only rotational structure characteristic of K *a Å 0 and the absence of nearby Ka subbands terminating on Ka ú 0 put an approximate upper limit on the height of the barrier to linearity. HCBr is also interesting because the lowest singlet and triplet states are thought to lie very close in energy (8). In fact, in our earlier work, we argued that rotational structure in the spectrum implied that rotational levels in the ground X˜ 1 A * (0, 0, 0) level were perturbed by triplet state levels, indicating an extremely small singlet– triplet separation. In the present work, we report measurement of the 2 10 band system of HCBr and we have also recorded the 2 20 band of the same A˜ 1 A 9 –X˜ 1 A * transition in DCBr. The spectra indicate that the molecule is bent in the upper state of both of these bands, although subbands involving levels with K *a ú 0 have not yet been completely rotationally analyzed. The analysis of the new spectra of HCBr brought into question some rotational assignments in the previous work (12), which have been corrected in the present study. It is now clear that there are no energy level perturbations in the ground state, X˜ 1 A * (0, 0, 0), of HCBr. Rotational constants extracted from all the currently available high-resolution spectra of bromomethylene and deutero-bromomethylene have been used to estimate a structure for the molecule. Using the position of the HCBr A˜ 1 A 9 (0, 1, 0) band origin in combination with the band origins of more highly excited bending levels of the A˜ 1 A 9 state measured previously (12, 13), the height of the barrier to linearity for the A˜ 1 A 9 state has been determined. II. EXPERIMENTAL DETAILS

The experimental setup used in the present study has been described previously (12). Briefly, bromomethylene radicals were generated by the 193-nm laser photolysis of bromoform, CHBr3 , (CDBr3 for DCBr) in a flow system. The absorption cell used in this experiment was a single-pass, 1.5-m-long, pyrex tube in which collinear photolysis and probe beams were overlapped. The pressure of bromoform in the absorption cell was approximately 25 mTorr. A flow of argon ( Ç20 sccm) was introduced at the ends of the absorption cell to minimize deposit of photolysis products on the windows. However, a dark film formed over time that necessitated approximately hourly cleaning of the windows. The argon also served to enhance vibrational and rotational cooling (to approximately 300 K, the temperature of the cell wall) of the radical product. The whole system was pumped by a mechanical pump via a liquid nitrogen cold trap. The pumping speed was adjusted to maintain a total pressure of approximately 600 mTorr in the absorption cell. More recently, we have recorded sections of the spectrum, using four to five times longer effective absorption

path lengths obtainable in a multipass absorption cell based on the design of Trutna and Byer (14–16). In all experiments, a weak blue-purple glow along the length of the photolysis laser beam could be seen by eye when the room lights were turned off. We tentatively ascribe this to fluorescence from CBr (17, 18). Laser transient frequency modulation spectroscopy (19) was used to detect the weak absorption spectrum. The nearinfrared probe laser beam was provided by the output of an Ar ion laser pumped Ti:sapphire ring laser. The beam was phase modulated at 200 MHz using an electro-optic modulator (Quantum Technology). The power in each of the firstorder sidebands was set at approximately 20% of the carrier. The phase-modulated beam was directed through the absorption cell and then focused onto a fast silicon PIN photodiode. Neutral density filters were used to limit the power on the detector to 10 mW, which when converted across 50 V, corresponds to approximately 300 mV for the 0.6-mA/mW sensitivity of our detector. After filtering, to eliminate low frequencies and amplification, the signal was demodulated at 200 MHz in a double-balanced mixer referenced to the 200-MHz oscillator. The mixer output was filtered to eliminate frequencies above 75 MHz and averaged using a digital oscilloscope (LeCroy 9300 series), controlled by a personal computer. Spectra were typically recorded by incrementing the probe laser frequency in 100 MHz steps and averaging 20 photolysis laser shots at each step. To permit subsequent study of the time dependence of the spectrum, the entire transient absorption waveform was recorded using 100 time points and the averaged waveform stored in the computer for every probe laser wavelength step. Spectra were displayed as the difference between the demodulated power before and after the arrival of the photolysis laser pulse. We predicted the expected position of the HCBr 2 10 band from previous observations of the 2 20 band (12) and higher bands (13) and scanned the region between 11 935 and 12 030 cm01 . Based on the laser-induced fluorescence spectra of HCBr recorded by Xu et al. (13) and the HCBr bands that we have measured at high resolution, we crudely estimated the band origin for the bromomethylene singlet system to be at Ç11 100 cm01 . Then, assuming a bending frequency of Ç625 cm01 for DCBr (13), we predicted that its 2 20 band should be observed at Ç12 350 cm01 . We accordingly searched for spectra at wavenumbers close to this and immediately detected rotational structure similar to that seen for HCBr. In total, the region from 12 330 to 12 400 cm01 was scanned for DCBr. III. RESULTS AND ANALYSIS

1. HCBr 2 20 and 2 10 Bands The upper panel of Fig. 1 illustrates the entire region of the 2 10 spectrum of HCBr recorded. Compared with previous observations at shorter wavelengths in the 2 20 band region (12), the present spectrum is more complex, with additional

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turbations in either state involved in the transition and, in particular, there is no evidence of the perturbations found for HC 81Br at J Å 20 and 21 in the 2 20 band of the molecule (12). In that previous study the rotational quantum number assignments implied that these perturbations were in the electronic ground state and their nonexistence in the present band means that either (i) the present spectrum originates from a different X˜ 1 A * state vibrational level or (ii) the previously reported perturbations are in fact in the upper level and the rotational transition assignments for the 2 20 band are in error. We consider it most unlikely that the lower state of the current band is different from that observed previously because the time dependence of the absorptions is identical in both cases and the experimental conditions used favor vibrational and rotational relaxation. Therefore only the second option above seems tenable, and this was confirmed after some analysis when a consistent set of rotational assignments, leading to identical lower state rotational constants for the two bands, was obtained. Revision of the analysis of the previously observed band led to Q-branch transitions with J ¢ 3 being reassigned. In the upper panel of Fig. 3 the new assignments in the Q(20) region are shown. In the lower panel, the P-branch transitions, which terminate at the same upper state rotational levels, are shown. The distinctive 79 Br level splittings in the P-branch lines are not as clear because of line overlap but, with hindsight, can be recognized in the spectrum shown and also, although not shown, in the corresponding R-branch transitions. These observations mean that the perturbed level is the upper one, because PFIG. 1. Overall views of the 2 10 band of HCBr (top) and the 2 20 band of DCBr (bottom). The main rotational branches are indicated. Note that both of these bands have more complex rotational band structure than the 2 20 band of HCBr (12), where transitions terminating in K a9 Å 0 dominate the spectrum.

rotational branch structure clearly visible. The strongest absorptions, around 11 960 cm01 correspond to Q-branch lines in the Ka Å 0 R 1 subband. This was the only subband with appreciable intensity observed in the previous work and the only one expected in this region if this level in the upper state of the transition has the rotational structure of a linear molecule. To shorter wavelengths, the spectrum in Fig. 1 shows additional Q-branch structures, tentatively assigned to the Ka Å 1 R 0 subband and the forbidden Ka Å 1 R 1 subband. Here we consider just the rotational assignments of the Ka Å 0 R 1 subband; work on the shorter wavelength structure is currently in progress and will be the subject of a future report. A section of the Q branch in the Ka Å 0 R 1 region is shown in Fig. 2. The 79Br, 81Br isotope doublet structure is characteristic of the two naturally occurring bromine isotopes, which have approximately equal natural abundances. The regular rotational structure shows little evidence of per-

FIG. 2. Section of the 2 10 band of HCBr near the pQ1-branch origin. Note the regular Br isotope doublet structure. The stronger absorptions to the red of the Q branch, near P(1) likely are caused by the Ka Å 1 R 2 subband but have yet to be assigned in detail. The small discontinuities in the spectrum comprise wavelengths not covered, due to incomplete overlaps of short scans during the data acquisition.

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usual centrifugal distortion constants. The results of the fit are summarized in Table 2. The overall standard deviation of the fit was 0.96, relative to an estimated uncertainty of 0.01 cm01 for measurements derived from well-resolved lines. Finally, the ground state rotational and centrifugal distortion constants were held fixed and the assigned spectra fit to a model where the upper state K *a Å 0 energies were represented by F *J Å n0 / BU J(J / 1) 0 DJ[J(J / 1)] 2 ,

FIG. 3. Section of the 2 20 band of HCBr showing (top) perturbations in the Q-branch region that cause splitting of the rotational lines. The same splittings are evident in the appropriate section of the P branch of the same band (lower) indicating that the perturbations causing them are in the upper state of the transition. Asterisks denote transitions of the HC 79Br isotopomer.

and R-branch transitions originate from a different Ka-asymmetry doublet component of the lower state than the Qbranch transitions. A complete set of assigned transitions for both the observed HCBr bands is given in Table 1. For the 2 20 band, numerous perturbations can be recognized as shifts and splittings in the spectral lines. The 2 10 band is relatively unperturbed, although the band is weaker presumably due to a smaller Franck–Condon factor and possesses more complicated rotational structure due to the presence of rotational branches with K *a ú 0. To confirm the assignments, ground state combination differences were extracted from the data for both bands and checked for consistency. Ground state rotational constants were derived by fitting these combination differences to a pseudolinear molecule rotational hamiltonian for K 9a Å 1: F 9J Å BU J(J / 1) 0 DJ[J(J / 1)] 2 1 { 2

F

G

(B 0 C) J(J / 1) 0 2dJ[J(J / 1)] 2 . 2

[1]

Here, BV Å (B / C)/2, and (B 0 C)/2 are combinations of the usual rotational constants, and DJ and dJ are the

where the rotational and centrifugal distortion constants now refer to the upper state and n0 is the band origin. The fitted band origins and rotational constants for the two isotopomers are given in Table 3. The overall standard deviations of the fits were somewhat larger than expected purely on the basis of the expected measurement errors, estimated to be only {0.007 cm01 . We attribute this to the combined effects of many small perturbations in the excited state rotational level positions. The fact that the ground state combination differences were fit to within the expected measurement uncertainty argues that there are no systematic measurement errors and that the ground state is unperturbed. It should be noted that the different weightings assigned to transitions (see footnotes to Table 1) in these fits means that the fit standard deviations, given in Table 3, do not accurately reflect the relative sizes of the perturbations observed in the different excited state vibrational levels. Some isolated transitions were recorded in greater detail, i.e., using a smaller frequency step size and with more averaging per point, and the resulting signals were accurately phase corrected and integrated (20, 21) to obtain Doppler broadened absorption lineshapes. The experimental lines were found to be broader when compared with the theoretically simulated lineshapes for a 300 K sample temperature. We suspect that these broader lineshapes are caused by unresolved bromine quadrupole splittings. 2. DCBr 2 20 Band A survey view of the observed DCBr spectrum is shown in the lower panel of Fig. 1. The band has similar rotational branch structure to the 2 10 band of HCBr shown in the upper panel but is more compressed. A section of the low-J Qbranch region is shown in the upper panel of Fig. 4. The rotational structure, especially that assigned to DC 81Br, is more strongly perturbed than was seen in either band of HCBr. The assignments here were confirmed by detecting the analogous perturbations in the P and R branches of the spectrum. An example is shown in the lower panel of Fig. 4, where the equivalent irregularities in the spacings to those observed in the Q branch near J * Å 13 are observed for the P-branch lines terminating in the same upper state rotational levels. The complete set of assigned lines in the Ka Å 0 R 1 subband of this DCBr transition are given in the last two columns of Table

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TABLE 1 Observed Lines in the 2 20 and 2 10 Bands of HCBr and the 2 02 Band of DCBr (in cm01 ) a

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TABLE 1 —Continued

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TABLE 2 Ground State Molecular Constants for HCBr and DCBr (in cm01 ) a

1. The data were reduced in the same way as those for HCBr, and the results are summarized in Tables 2 and 3. IV. DISCUSSION

1. Vibrational Assignments Previously, arguments were put forward for assigning the HCBr 2 02 band as originating from the zero point vibrational level of the ground state of the radical (12). Briefly, the experimental conditions used favor vibrational and rotational relaxation on the timescale of the observations. With lower buffer gas pressures, the measured transient absorption waveforms exhibit a slower rise, indicative of the slowing of collisional relaxation processes. The vibrational normal mode frequencies in the ground state of HCBr are ¢670 cm01 (22), and based on this, the lowest-lying excited vibrational levels of DCBr are expected to lie ¢600 cm01 above the zero point level. Under ambient conditions such levels are unlikely to be significantly populated in our experiment. The rotational analysis confirms that both of the observed HCBr bands (2 02 and 2 10 ) originate from the same vibrational level. With regard to the upper state vibrational levels, the position of the 2 10 band is consistent with all previous assign-

ments. We can use the present data together with that of Xu et al. (13) to construct a plot of DGv/1 / 2 against 1/2( nv / nv/1 ) (23) where DGv/1 / 2 is the observed spacing between the bands with £2 Å £ and £ / 1, and nv ( nv/1 ) are the observed band positions. The spacing between the 2 20 and 2 10 bands is 829 cm01 , and Xu et al. (13) found spacings of 842 and 863 cm01 between the 2 50 , 2 60 , and 2 70 bands, respectively. The minimum of such a plot is found to occur for the (hypothetical) vibrational level at 13 590 cm01 , which, as Dixon has shown (23), is the approximate location of the barrier to linearity. The same plot provides an estimate of the 0 00 origin band of the A˜ R X˜ band system to be at approximately 11 120 cm01 . In addition, these results imply that the A˜ (0, 2, 0) level lies just below the barrier, i.e., its rotational structure must be that of a bent molecule, despite the original assignment of only a Ka Å 0 R 1 subband, which would suggest linear molecule behavior. Indeed a reexamination of the previously recorded spectrum shows that considerable, additional weak structure exists in the assigned 2 20 band above 12 820 cm01 (12), which is likely due to the Ka Å 1 R 0 subband. If this assignment is correct it would imply a very large (40–60 cm01 ) effective A rotational constant in the (0, 2, 0) level, which is consistent

TABLE 3 Upper State Molecular Constants for HCBr and DCBr (in cm01 ) a

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the data for HCBr above. Therefore the 2 20 vibrational assignment proposed here for DCBr is less secure. It is consistent with the HCBr spectrum, but not with all of the assignments of Ref. (13). 2. Rotational Assignments

FIG. 4. Low-J section of the Q-branch of the DCBr 2 20 band showing extensive perturbations. To make rotational assignments, P- and R-branch transitions showing the analogous shifts and splittings had to be identified. The lower panel shows a section of the corresponding P branch indicating the unevenness of the Br isotope structure here, in contrast with most of the other spectra.

with the expected large bond angle and floppy nature of the bending vibration at this energy, just below the barrier to linearity in the A˜ state of HCBr. Xu et al. (13) identified many stretch–bend combination bands in their low-resolution spectra. Based on their assignments, we would predict the 2 103 10 band to be at 12 740 cm01 , at lower frequency than scanned previously (12). However, carefully examining our spectrum shows a weak, regular, progression of rotational lines at the very lowest wavenumbers scanned, which could be caused by high- J R- and Qbranch lines of the predicted combination band. The pure stretching band 3 10 is predicted to lie close to 11 900 cm01 , and this region has yet to be searched. For DCBr, we have tentatively assigned the observed band as 2 20 based on its position relative to the estimated origin of the band system. The assignment implies a harmonic frequency of v2 Å 615 cm01 in DCBr. Xu et al. (13) measure 619, 634, and 649 cm01 for the spacings between bands assigned as 2 70 , 2 80 , 2 90 , and 2 10 0 , respectively. Although the partially resolved Br isotope splittings (13), Ç1 cm01 in 2 70 , are in reasonable agreement with the 0.34-cm01 spacing observed in the 2 20 band, the apparent position ( Ç15 580 cm01 ) of the band in Fig. 3 of Ref. (13) assigned as 2 70 cannot be reconciled with the band origin estimated from

Following the reassignments in the Q branch of the 2 20 band of HCBr, a consistent set of rotational energy levels for the ground state of the radical was obtained. The K-type asymmetry doubling in the lower level allows determination of the difference (B 0 C) of the two end-over-end rotational constants as well as their average. Compared with the previous results (12) the present rotational constants show a regular bromine isotope effect and there is no evidence for either local or global perturbations in this level. Although the upper state rotational levels do show some perturbations in HCBr, these are not large and we can estimate average rotational constants for both the A˜ (0, 1, 0) and A˜ (0, 2, 0) levels (see Table 3). There is a slight increase in the average rotational constant BV upon electronic excitation, consistent with a slight decrease in the C–Br bond length. The difference between the BV value for £*2 Å 1 compared with £*2 Å 2 is very small, consistent with their assignments to bending levels near the top of the barrier to linearity. For the 2 20 band of DCBr, the ground state is also unperturbed, as it must be on the basis of the HCBr analysis, but, in contrast to HCBr, we find a very small decrease in BV on electronic excitation. We examine this behavior in the following section. In the excited state, there are some severe perturbations in the rotational structure, as can be seen in Fig. 4. Because the estimated vibrational frequencies in the A˜ state of DCBr ( v *2 É 615 cm01 and v *3 É 769 cm01 (13)) seem to preclude local interactions with other vibrational levels of this state, the perturbations must be caused by interactions with the background of X˜ 1 A * and a˜ 3 A 9 levels. We are currently attempting to assign the observed rotational structure in the other subbands present in the spectrum and have tentative assignments for the Ka Å 1 R 0 subband, which will enable a determination of (B–C) for the upper state vibrational level as well as a precise determination of A for both upper and lower state vibrational levels. 3. Geometry To experimentally determine structural parameters for HCBr, information on the A rotational constant is required. From our preliminary analysis of the additional rotational branch structure, we have made estimates of A for the 81Br isotopomer of HCBr and both isotopomers of DCBr. They are derived from the positions of the Ka Å 1 R 0, Ka Å 1 R 2, and Ka Å 0 R 1 subbands in the DCBr spectrum and the corresponding subbands of the 2 01 band of HCBr. They must be considered approximate, particularly in the case of HCBr, because no correction has been made for the centrifugal distortion constant DK , which could be large in the upper state of

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TABLE 4 A Comparison of the Experimentally Determined Rotational Constants and Values Calculated from a Fit to the Structural Parameters for the X˜ 1 A* State of HCBr a

both these bands. The B and C constants also refer to Ka Å 1 in the ground state; however, all indications are that the molecule is rigid and well behaved in this level so corrections due to centrifugal distortion parameters should be small. Noting these concerns, the appropriate rotational constants for the ground states of the various isotopomers have been collected in Table 4. They were used in a least-squares fit to the three structural parameters with the resulting values given in Table 5. The bond lengths and angle from recent ab initio calculations are also given in Table 5 for comparison (24– 26). By far the best agreement is with the results of Gobbi and Frenking (26) who predict both the rCH and UHCBr values to within experimental error. Although rCBr is predicted to be longer than experiment, the value is in far better agreement than with those of the two other studies. As a simple way of examining the effects of shortening the C–Br bond length in the excited state we estimated that rCBr and rCH would reduce by approximately 4–5% from the ground state values consistent with the changes observed in ˚ , rCBr Å CH2 (6) and HCCl (10). Values of rCH Å 1.08 A ˚ , and a bond angle UHCBr Å 1437 reproduced the BV 1.77 A value of 0.437 cm01 obtained for the (0, 2, 0) level of HCBr.

More interestingly it predicts an A value for that state of 47.1 cm01 , which is not inconsistent with the observation that the origin of the weak Ka Å 1 R 0 subband of the 2 20 band probably lies at approximately 12 835 cm01 , well above the assigned Ka Å 0 R 1 subband. In addition, the A˜ state BV value (0.378 cm01 ), which this structure predicts for DCBr, although not in accurate agreement with the value observed in the A˜ (0, 2, 0) level, is smaller than that determined for the ground state. This shows that C–Br bond shortening is compatible with a decrease in rotational constant provided that the bond angle opens up. A definitive experimental determination of the structure of this radical will have to await a precise estimate of the A rotational constants for both HCBr and DCBr. V. SUMMARY AND CONCLUSIONS

The observation of two new bands of the A˜ 1 A 9 R X˜ 1 A * electronic transition of bromomethylene, and the reassignment of the previously observed band, has produced an improved understanding of both the ground and the excited singlet states. The unperturbed nature of the rotational levels

TABLE 5 A Comparison of Experimentally and Theoretically Determined Structural Parameters for the X˜ 1 A* State of HCBr

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of the vibrationless ground state levels of both HCBr and DCBr suggests that the triplet state lies above the vibrationless ground state level. This is in agreement with the recent theoretical calculations and low-resolution studies, which suggest values ranging from 1400 to 4500 cm01 for DEt ,s (8, 22, 24–27). To obtain a more accurate value will require searching for perturbations in vibrationally excited levels of the ground state. With the increase in sensitivity we have recently achieved using a multipass laser configuration in combination with a reduction in vibrational cooling (lower buffer gas pressure) it may be possible to record the absorption spectra of hot bands using our frequency modulation absorption spectrometer. More fruitful techniques may include dispersed fluorescence spectroscopy, which could conceivably produce a map of the ground state vibrational levels, or stimulated emission pumping (28). However, recent attempts at recording dispersed fluorescence spectra of a similar species HCCl in our laboratory have not yet proved successful (29). The excited state picture is clearer; the estimates obtained for the energies of both its vibrational origin level and the barrier to linearity should both be quite accurate, as should predictions of the positions of other stretching vibrational levels. In the near future we hope to be able to confirm the position of the band origin by recording its position using a near-infrared diode laser as a source of the probe radiation (14) and also observe the 2 30 band, which will improve the precision of the estimate for the barrier to linearity. It should also be possible to record the 2 10 band of DCBr to confirm that we have correctly assigned the present band, particularly because its vibrational assignment is at odds with a previous study (13). An added dimension will also be obtained if we are able to observe transitions to excited C–Br stretching levels, as was possible for the corresponding C–Cl stretching levels of HCCl (11). ACKNOWLEDGMENTS Work at Brookhaven National Laboratory was carried out under Contract DE-AC01-76CH00016 with the U.S. Department of Energy and supported by its Division of Chemical Sciences, Office of Basic Energy Sciences. Work at MIT was supported by Contract DE-FG02-87ER-13671.

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