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    J. Phys. Chem. A 2008, 112, 466-471
    
    Stimulated Emission Pumping Spectroscopyof the [X]1A′ State of CHF ˜
    Calvin Mukarakate,† Chong Tao,† Christopher D. Jordan,‡ William F. Polik,*,‡ and Scott A. Reid*,†
    Department of Chemistry, Marquette UniVersity, Milwaukee, Wisconsin 53201-1881, and Department of Chemistry, Hope College, Holland, Michigan 49422-9000 ReceiVed: September 4, 2007; In Final Form: October 24, 2007
    
    We have recorded stimulated emission pumping (SEP) spectra of the A1A′′ f X1A′ system of CHF, which ˜ ˜ reveal rich detail concerning the rovibronic structure of the X1A′ state up to ∼7000 cm-1 above the vibrationless ˜ level. Using several intermediate A1A′′ state levels, we obtained rotationally resolved spectra for 16 of the 33 ˜ levels observed in our previous single vibronic level (SVL) emission study (Fan et al., J. Chem. Phys. 2005, 123, 014314), in addition to one new level. An anharmonic effective Hamiltonian model poorly reproduces the term energies even with the improved set of data because of the extensive interactions among levels in a given polyad (p) having combinations of ν1, ν2, ν3, which satisfy the relationship p ) 2ν1 + ν2 + ν3. However, the precise A rotational constants determined from the SEP data were invaluable in clarifying the assignments for these strongly perturbed levels, and the data are well reproduced using a multiresonance effective Hamiltonian model. The derived vibrational parameters are in good agreement with high level ab initio calculations. The experimental frequencies were combined with those of CDF to derive a harmonic force field and average (rz, r z ) structures for the ground state. e
    140 cm-1 was derived from photoelectron spectra of CHF28.) An anharmonic effective Hamiltonian model well reproduced the experimental term energies of CDF. However, the spectra of CHF were complicated by Fermi resonances among levels in a given polyad (p) having combinations of ν1, ν2, and ν3, which satisfied the relationship p ) 2ν1 + ν2 + ν3.60 Because of the extensive anharmonic interactions, the assignments for many levels of CHF were considered tentative. In this study, we have applied rotationally resolved stimulated emission pumping (SEP) spectroscopy to probe the vibrational mixing in the X1A′ state and identify spin-orbit interactions ˜ with the low lying a3A′′ state, which have previously been ˜ observed in the A1A′′ state using quantum beat spectroscopy.56 ˜ Suzuki and Hirota reported emission and high-resolution SEP spectroscopy from the 00 level of CHF,50 identifying a resonance between 11 and 2131 and an unusually small anharmonic C-H stretching frequency [2643.0393(26) cm-1]. However, their study investigated only a few levels up to 3000 cm-1 above the vibrationless level of the X1A′ state. Building upon this work ˜ and our previous studies using fluorescence excitation56 and SVL emission60 spectroscopy, we have extended the SEP studies up ˜ to ∼7000 cm-1 above the vibrationless level of the X1A′ state. The precise A rotational constants determined from the SEP data have clarified the assignments for many levels, and we have modeled the data using a two-resonance effective Hamiltonian model. The derived vibrational parameters are in excellent agreement with high level ab initio calculations, and the experimental harmonic frequencies are combined with those of CDF to derive a harmonic force field and average (rz) structure for the ground state. Experimental Procedures
    * Corresponding author. E-mail: scott.reid@mu.edu (S.A.R.); polik@ hope.edu (W.F.P.). † Marquette University. ‡ Hope College.
    
    Introduction Carbenes are important reactive intermediates that are found in organic synthesis, combustion, thermal decomposition of small organic molecules, and atmospheric and interstellar chemistry.1-13 The chemistry of carbenes is fascinating because the divalent carbon gives rise to low-lying singlet and triplet states with similar energies but distinct chemical reactivity. A central issue in carbene chemistry, and the focus of many theoretical and some experimental studies over the years, has been the precise determination of the singlet-triplet energy gap, ∆ETS. 14-40 The monohalocarbene CHF is the smallest carbene with a singlet ground state, and is thus a prototype for exploring the spectroscopy, dynamics, Renner-Teller effect in and electronic structure of singlet carbenes.14-34,41-59 Using laser spectroscopy, recent progress has been made on the experimental side in probing the vibrational structure of the X1A′ state and spin˜ orbit coupling for the set of monohalocarbenes HCX (X ) F, Cl, and Br).34,36-40,60-65 For example, SVL emission spectra of CHCl, CDCl, CHBr, and CDBr show transitions to triplet levels, which borrow intensity from nearby levels of the X1A′ ˜ state.36-40,60-65 Experimental estimates for the magnitude of ∆ETS derived from the emission studies are in very good agreement with theoretical predictions.36-40,61-65 We previously reported single vibronic level (SVL) emission spectra of CHF and CDF which mapped the vibrational levels up to ∼10,000 cm-1 above the vibrationless level of the X1A′ ˜ state, encompassing the region where the triplet origin is predicted. (Theoretical values for ∆EST range from 4617 to 5537 cm-1,14-22,24-27,29-33 while an experimental value of 5210 (
    
    The apparatus, pulsed discharge nozzle, and data acquisition procedures have been described in previous studies.55-66 CHF was generated by a pulsed electrical discharge through a 1% to
    
    10.1021/jp077108m CCC: $40.75 © 2008 American Chemical Society Published on Web 01/03/2008
    
    SEP Spectroscopy of CHF 2% mixture of CH2F2 (Aldrich, 99.9%) in argon (BOC gases, 99.999%) that was premixed in a 1 L stainless steel cylinder. The mixture was continuously expanded through a pulsed nozzle at backing pressures of ∼2 bar. The discharge was initiated by a +1 kV pulse of ∼100 µs duration, through a current-limiting 100 kΩ ballast resistor. The timing of the lasers, nozzle, and discharge pulse firing was controlled by two digital delay generators (Stanford Research Systems DG535), one of which generated a variable width gate pulse for the high voltage pulser (Directed Energy GRX-1.5K-E). The pump laser beam (was generated from a tunable dye laser (Spectra Physics PDL-3, line width ∼0.3 cm-1) pumped by the second harmonic of a Nd:YAG laser (Powerlite 7010); this beam excited specific rovibrational transitions in the A r X system. After a delay of approximately 400-500 ns, a ˜ ˜ counter propagating beam from a second dye laser (LambdaPhysik Scanmate 2E, line width ∼0.15 cm-1), pumped by the second or third harmonic of a Nd:YAG laser (Continuum NY61), stimulated emission to the X state or excited transitions in ˜ the B r A system.66 The laser beams were not focused, and ˜ ˜ typical pulse energies were ∼1-2 mJ for the pump and ∼10 mJ for the dump in an ∼3 mm diameter beam. A mutually orthogonal geometry of lasers, molecular beam, and detector was used, where the two overlapped lasers crossed the molecular beam at ∼1 cm downstream. Fluorescence was collected and collimated by a 2 in. diameter, f/2.4 plano-convex lens and filtered via an appropriate cutoff filter (Corion or Edmund Scientific) prior to striking a photomultiplier tube detector (PMT; Oriel 77348) held at -700 V. Typically, longpass filters with cut-on wavelengths longer than the dump wavelength were used; however, in experiments where the 11 0 21 level was pumped, a short-pass filter with a cutoff wave0 length of 600 nm was used. The signal from the PMT was fed to dual boxcar integrators (Stanford Research Systems SR250), and the SEP signal was obtained by integrating a portion of the fluorescence decay before and after the dump laser pulse. The ratio of signal from each gate was recorded to remove the effect of shot-to-shot fluctuations in the signal. Typically, 10 shots were averaged at each wavelength step of 0.002 nm. The wavelength of the first laser was calibrated using the well-known A r X spectroscopic constants.56 The second laser ˜ ˜ was calibrated using optogalvanic transitions in Ar or Ne; we estimate an absolute calibration error of less than 0.5 cm-1. The SEP lines were fit to a Gaussian line shape function, using Origin 7.5 software, and the transition frequencies were leastsquares fit to an asymmetric top Hamiltonian to derive the vibrational term energy and rotational constants, using the upper state constants determined in our previous work.56 Results and Discussion We obtained SEP spectra from four intermediate A1A′′ state ˜ levels, 00, 22, 2131, and 1121, being careful to select levels that 0 0 00 00 were not perturbed by Renner-Teller or spin-orbit interactions.56 Figure 1 displays typical spectra, which feature fluorescence depletions of 10-20%. Note that the broader peaks in Figure 1 reflect upward transitions to the B state, as we ˜ recently reported.66 The pump and dump transitions follow C-type selection rules (∆J ) 0, (1; ∆Ka ) (1; ∆Kc ) 0, (2). Consequently, a maximum of five lines are observed when pumping transitions with Ka′ ) 1 in the A state, as shown in ˜ Figure 1. The ∼20 K rotational temperature of our beam limited the pump transitions to excited-state levels with Ka′ ) 0, 1, and 2.
    
    J. Phys. Chem. A, Vol. 112, No. 3, 2008 467
    
    Figure 1. SEP spectra (inverted for clarity) for CHF X (0,1,0). These ˜ correspond to pumping Ka′ ) 1 in the A state; the upper panel displays ˜ the spectrum obtained by pumping rotational line J′ ) 2 and the lower panel is for pumping J′ ) 3. The spectra reflect a 10 shot average, and the x-axis labels the shifts in frequency from the excitation line.
    
    TABLE 1: Vibrational Term Energies and Rotational Constants (in cm-1) for CHF Derived from SEP Spectroscopy
    assignmenta (0,1,0) (0,0,2) (0,1,1) (1,0,0) (0,2,0) (0,1,2) (1,0,1) (1,1,0) (0,2,1) (0,3,0) (1,0,2)e (0,2,2) (2,0,0) (1,2,0) (0,4,0) (2,1,0) (1,3,0) (0,5,0) Ab 15.96 15.46 15.74 15.32 16.20 15.74 15.21 15.73 15.91 16.48 15.27 15.23 15.11 16.06 16.81 15.36 16.32 17.16 Bc 1.180 1.145 1.165 1.194 1.171 1.149 1.158 1.171 1.162 1.167 1.148 1.153 1.174 1.197 1.165 1.199 1.161 1.166 band origind 1403.2 2364.3 2568.3 2642.9 2812.7 3729.4 3835.7 3936.5 4017.4 4220.7 5008.5 5075.6 5134.7 5295.6 5617.3 6372.0 6640.1 7001.7
    
    a Approximate assignment based on the two-resonance effective Hamiltonian model (Model 3; see text). b Estimated uncertainty of ( 0.04 cm-1. c Estimated uncertainty of ( 0.010 cm-1. d Estimated uncertainty of ( 0.5 cm-1. e Not observed in previous emission studies.
    
    Table 1 presents the fit parameters for the measured bands, which are in good agreement with previous data for highresolution SVL emission49 and SEP spectroscopy.50 We measured SEP spectra for 17 levels, including one level that was not observed in our SVL emission study.60 For each level, the A and B ) (B + C)/2 rotational constants were determined. h Because the A constant is very sensitive to the quanta of bending excitation, as shown in Figure 2, it is helpful in clarifying the vibrational assignments. Initially, we performed a fit of the observed A constants to following the expression:
    
    Aυ1,υ2,υ3 ) A0 +
    
    ∑ Riυi i
    
    (1)
    
    468 J. Phys. Chem. A, Vol. 112, No. 3, 2008
    
    Mukarakate et al. theory,70 from which the resonances were removed and treated explicitly.71,72 Note the good agreement between experiment and ab initio constants for each model, and the marked improvement in the fit for the one- and two-resonance models. The latter reproduces the term energies to within 2 cm-1, which is similar to the fit standard deviation obtained in our SVL emission study of CDF.60 Lists of assignments and fit deviations for each model are provided in Table 3. A consequence of the strong Fermi resonance interactions in CHF, evidenced in the large third-order constants k122 and k123 (Table 2), is extensive mixing of the levels within a given polyad. This is readily shown in the mixing coefficients, which describe the zeroth-order composition of the eigenstates, which were determined from our analysis. For example, in the polyad with p ) 4, which contains seven levels and extends over 600 cm-1, the purest state is the highest lying member at 5617.3 cm-1, which comprises ∼70% 24 and ∼27% 1122, with smaller contributions from other zeroth-order states. This finding that the bending overtone in a given polyad most closely resembles the pure normal mode state is quite general and explains the good linear correlation shown in Figure 2. A detailed listing of the wavefunctions produced from the POLYAD program73 is provided in the Supporting Information. Converting the fit frequencies to harmonic frequencies using the relationship:67
    
    Figure 2. Dependence of the A rotational constant on the bending quantum number for the pure bending and combination states in CHF (X1A′), derived from the SEP data. For comparison, previous results ˜ from Hirota and co-workers50 are shown.
    
    which returned the parameters (in cm-1): R1 ) -0.26(9), R2 ) 0.27(5), R3 ) -0.21(7), and A0 ) 15.69(15). The standard deviation of this fit (0.20 cm-1) was far in excess of our experimental uncertainty (∼0.04 cm-1), reflecting the fact that any vibrational assignment of the levels in CHF is only approximate, as anharmonic mixing destroys the traditional normal mode labels. The SEP and SVL emission data were combined in a nonlinear least-squares fit to several different effective Hamiltonian models. Model 1 was the standard anharmonic model (Dunham expansion) referenced to the vibrationless level:67
    
    ωi ) ω0 - x0 i ii
    
    ∑xij0/2 j*i
    
    (5)
    
    G0(υ1,υ2,υ3) )
    
    ∑ υiωi0 + jgi,i)1 υiυjx0 ∑ ij i)1
    
    3
    
    3
    
    (2)
    
    where ω0 is the harmonic frequency of mode i, x0 is a diagonal i ij anharmonicity constant, and x0 is an off-diagonal or crossij anharmonicity constant. Resonance-effective Hamiltonian models (Models 2 and 3) were employed, which included one (k123) and two (k122 and k123) Fermi resonance matrix elements, respectively, of the form
    
    〈ν1,ν2,ν3|HF|ν1 - 1,ν2 + 1,ν3 + 1〉 ) k123[(ν1)(ν2 + 1)(ν3 + 1)/8]0.5 (3)
    and
    
    (note the typographical error in this formula given in our previous report60), we obtain (in cm-1) ω1 ) 2785.2(48), ω2 ) 1445.1(16), and ω3 ) 1211.2(27). In comparison, high level ab initio calculations give values (in cm-1) ranging from 2767 to 2815 for ω1, 1433-1448 for ω2, and 1171-1206 for ω3.33,54,60 We used the derived harmonic frequencies and reported centrifugal distortion constants of CHF50,74 and CDF46 to perform a normal-mode analysis, with the harmonic force field refined using the ASYM40 program of Hedberg and Mills.75 Table 4 presents the fit results and harmonic force constants. We were able to derive the entire force constant matrix save one off-diagonal element, f12, which was set to zero in the fit. The harmonic corrections to the CHF and CDF rotational constants determined from the force field were used in a leastsquares analysis to compute an average (rz) structure for CHF. In our analysis, allowance was made for a 0.0035 Å Laurie contraction of the C-H bond upon deuteration, calculated using the following expression:76,77
    
    〈ν1,ν2,ν3|HF|ν1 - 1,ν2 + 2,ν3〉 ) k122[(ν1)(ν2 + 1)(ν2 + 2)/8]0.5 (4)
    The fit parameters and standard deviations for all three models are given in Table 2, along with the results of ab initio calculations from a quartic potential energy surface computed at the CCSD(T)/aug-cc-pVQZ level. The MOLPRO program68 was used to optimize the geometry of CHF and compute energies of displaced geometries, which allowed the determination of the quadratic, cubic, and quartic force constants by numerical differences. A modified version of the SPECTRO program69 was then used to calculate the harmonic frequencies, anharmonic constants, and resonance constants from the computed equilibrium geometry and force field. The anharmonic constants were calculated using second-order perturbation
    
    δrz ) 3/2 aδ〈u2〉 - δK
    
    (6)
    
    where the difference in mean square displacement, 〈u2〉, and perpendicular amplitude correction δK were obtained from the force field, and the Morse anharmonicity parameter (a; units of Å-1) was estimated from data for the corresponding diatomic (C-X) molecules78 input into the following expression:79
    
    a ) 2/3 πωe
    
    6(Be) cµ Re + 3 ωe 2h(Be)
    
    (
    
    2
    
    )
    
    (7)
    
    An rez structure was also determined using the following relationship:80
    
    rz ) rz - 3/2 a〈u2〉 + K e
    
    (8)
    
    SEP Spectroscopy of CHF
    
    J. Phys. Chem. A, Vol. 112, No. 3, 2008 469
    
    TABLE 2: Comparison of the Derived Vibrational Parameters (in cm-1) for the X State of CHF for the Three Effective ˜ Hamiltonian Models Described in the Texta
    Model 1 parameter ω0 1 ω0 2 ω0 3 x0 11 0 x12 x0 13 x0 22 x0 23 x0 33 k122 k123 σ
    a
    
    Model 2 calc. 2670.6 1385.2 1196.6 -71.1 -118.8 10.1 17.1 -17.3 -9.9 exp. 2684.2(81) 1414.9(16) 1195.1(63) -62.1(40) -56.7(13) -0.5(52) -3.0(3) -7.7(9) -7.2(33) 83.5(32) 5.5 calc. 2662.4 1385.2 1196.6 -71.1 -127.0 1.9 17.1 -9.0 -9.9 99.2 exp.
    
    Model 3 calc. 2697.3 1402.7 1196.6 -71.1 -57.0 1.9 -0.4 -9.0 -9.9 -103.3 99.2 2706.0(39) 1413.7(9) 1200.1(21) -62.6(22) -37.9(25) 4.7(24) -7.8(6) -9.2(5) -8.9(11) -113.4(36) 95.0(17) 2.1
    
    exp. 2727(31) 1414(7) 1196(15) -81(17) -82(7) 11(13) -2.9(12) -0.4(27) -13(5)
    
    25.7
    
    Also shown are the parameters calculated at the CCSD(T)/aug-cc-pVQZ level.
    
    TABLE 3: Comparison of Assignments and Fit Deviations (in cm-1) for X State Levels of CHF for the Three Effective ˜ Hamiltonian Models Described in the Texta
    Model 1 Model 2 Model 3 term energy assignment O.-C.b assignment O.-C. assignment O.-C. 0 1192 1403.2 2364.3 2568.3 2642.9 2812.7 3729.4 3835.7 3936.5 4017.4 4220.7 5008.5 5075.6 5134.7 5220 5295.6 5411 5617.3 6299 6372.0 6477 6640.1 6798 7001.7 7679 7820 7987 8177 8384 8951 9336 9548 9749 (0,0,0) (0,0,1) (0,1,0) (0,0,2) (0,1,1) (1,0,0) (0,2,0) (0,1,2) (1,0,1) (1,1,0) (0,2,1) (0,3,0) (1,0,2) (0,2,2) (2,0,0) (1,1,1) (1,2,0) (0,3,1) (0,4,0) (0,2,3) (2,1,0) (1,2,1) (1,3,0) (0,4,1) (0,5,0) (0,3,3) (1,3,1) (0,4,2) (0,5,1) (0,6,0) (1,3,2) (0,5,2) (0,6,1) (0,7,0) 0 -9 8.3 -26.1 25.2 3.2 4.5 19.5 4.0 38.9 -18.6 -3.6 -1.9 78.1 -4.8 -51 3.2 -12 -6.1 -17 4.8 15 -23.7 -6 -2.3 1.0 -11 -41 3 -2 24 -3 13 10 (0,0,0) (0,0,1) (0,1,0) (0,0,2) (0,1,1) (1,0,0) (0,2,0) (0,1,2) (1,0,1) (1,1,0) (0,2,1) (0,3,0) (1,0,2) (1,1,1) (0,2,2) (2,0,0) (1,2,0) (0,3,1) (0,4,0) (0,2,3) (2,1,0) (1,2,1) (1,3,0) (0,4,1) (0,5,0) (2,2,0) (1,3,1) (1,4,0) (0,5,1) (0,6,0) (2,3,0) (0,5,2) (0,6,1) (0,7,0) 0 -4 8.7 -3.0 5.6 -2.7 5.0 5.1 -3.0 5.0 8.6 -3.2 0.1 -2.4 -4.3 2 0.5 2 -5.9 2 6.1 -3 2.0 -2 -2.6 -8 -6 -6 -3 -3 8 3 -1 8 (0,0,0) (0,0,1) (0,1,0) (0,0,2) (0,1,1) (1,0,0) (0,2,0) (0,1,2) (1,0,1) (1,1,0) (0,2,1) (0,3,0) (1,0,2) (0,2,2) (2,0,0) (1,1,1) (1,2,0) (0,3,1) (0,4,0) (0,2,3) (2,1,0) (1,2,1) (1,3,0) (0,4,1) (0,5,0) (2,1,1) (1,3,1) (1,4,0) (0,5,1) (0,6,0) (1,4,1) (0,5,2) (0,6,1) (0,7,0) 0 0 2.7 0.3 0.8 0.0 2.7 -0.7 2.1 0.9 1.8 -1.5 0.4 -1.1 2.2 -4 -3.3 0 -1.5 -2 1.8 1 1.3 -1 2.0 -2 1 -2 -1 -2 1 2 -1 2
    
    TABLE 4: Results of the Force Field Analysis of CHFa
    fitted data parameter ω1/cm-1 ω2/cm-1 ω3/cm-1 ∆J/MHz ∆JK/MHz ∆K/MHz δJK/MHz δK/MHz CHF observed 2785.2 1445.1 1211.2 0.1181 2.370 70.99 0.009859 1.684 f11(mdyn/Å) f22(mdyn/Å) f33(mdyn/Å) 1 2 3 CHF O.-C.a -1.7 0.7 0.6 -0.010 0.103 1.30 0.000512 0.266 CDF observed 2049.9 1214.3 1080.7 0.1004 1.478 25.58 0.01245 1.200 CDF O.-C. 1.7 0.6 1.6 0.0010 0.025 1.89 0.00065 0.190 0.0(fixed) 0.455(23) 0.644(23)
    
    force constants 4.255(10) f12(mdyn/Å) 1.345(10) f13(mdyn/Å) 6.718(10) f23(mdyn/Å) coordinate notation C-H stretch H-C-F bend C-F stretch
    
    a The centrifugal distortion constants were obtained from refs 47, 50, and 68. b O.-C., observed-calculated.
    
    an effort to improve the fit, we refit the A constants using eq 1 with effective quantum numbers based upon the wavefunction composition predicted by the POLYAD program. The fit standard deviation decreased by a factor of 2, yet was still twice our experimental uncertainty. In this regard, SEP spectra of the deuterated isotopomer CDF should be useful in obtaining a precise set of rotation-vibration constants for further structural refinement and modeling of the anharmonic CHF force field. Conclusions We have recorded stimulated emission pumping (SEP) spectra of the A1A′′ f X 1A′ system of CHF, which reveal rich detail ˜ ˜ concerning the rovibronic structure of the X1A′ up to 7000 cm-1 ˜ above the vibrationless level. Spectra were obtained from several intermediate A1A′′ state levels, 00, 22, 2131, and 1121, and we ˜ 0 0 00 00 obtained rotationally resolved spectra for 16 of the 33 levels observed in our previous single vibronic level (SVL) emission study,60 in addition to one new level. An anharmonic effective Hamiltonian model poorly reproduces the term energies because of the extensive interactions among levels in a given polyad (p) having combinations of ν1, ν2, and ν3, which satisfy the relationship p ) 2ν1 + ν2 + ν3. However, the precise A rotational constants determined from the SEP data aided in clarifying the assignments for these strongly perturbed levels,
    
    a This table includes data from both SEP (this work) and SVL emission60 spectroscopy. b O.-C., observed-calculated in cm-1.
    
    The derived structural parameters are rz(C-H) ) 1.118(1) Å, rz(C-F) ) 1.311(1) Å, rez (C-H) ) 1.104(1) Å, rez (C-F) ) 1.304(1) Å, and θ HCF) 103.1(1)°. In comparison, ab initio predictions give equilibrium structural parameters values ranging from 1.120 to 1.126 Å for r(C-H), 1.309 to 1.324 Å for r(CF), and 101.9 to 102.4° for θHCF.33,54,60 In principle, the vibration-rotation constants (R′s) determined from a fit of the i SEP data could be used to determine an re structure; however, this was not attempted because of the poor quality of the fit. In
    
    470 J. Phys. Chem. A, Vol. 112, No. 3, 2008 and the use of a multiresonance effective Hamiltonian model improved the fit standard deviation to near the limits of experimental uncertainty. The derived vibrational parameters are in good agreement with high level ab initio calculations; these frequencies were combined with those of CDF to derive a harmonic force field and average (rz) structure for the ground state. Despite a strongly perturbed vibrational structure in the ground X1A′ state, we have not obtained in this work any ˜ evidence for spin-orbit interactions with the low-lying a3A′′ ˜ state. Such perturbations have been observed, however, in SVL emission spectra of the heavier monohalocarbenes, and SEP spectroscopy should prove useful in interrogating the structure of the triplet state and spin-orbit mixing in these systems. Acknowledgment. S.A.R. gratefully acknowledges support of this research by the National Science Foundation under grants CHE-0717960 and CHE-0353596. W.F.P. and C.D.J. gratefully acknowledge the National Science Foundation (CHE-0520704 and CHE-0624602) and the Howard Hughes Medical Institute for computing resources and stipend support to carry out this research. We thank Dr. Scott H. Kable for experimental assistance and helpful advice regarding this work, and John L. Davisson for assistance with the analysis. Supporting Information Available: Wave functions for the observed eigenstates derived from the polyad two-resonance model. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
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