Zero-point Level Research Articles

High resolution spectroscopic studies of 1-(1-naphthyl)ethylamine in S 0 and S 1: exploring the dependence of circular dichroism on conformational structure

Rotationally resolved spectra of gas-phase samples of 1-(1-naphthyl)-ethylamine (NEA) and amine deuterated forms have been obtained in the microwave and ultraviolet regions, with the isotopomers initially prepared in their zero-point vibrational levels by cooling in pulsed jet and molecular beam supersonic expansions. A single parameter set that includes inertial parameters and 14N nuclear quadrupole constants has accounted for nearly all transitions observed in the Fourier-transform microwave spectrum at 2 K and indicates the presence of only one geometrical isomer in the jet-cooled expansion. The rotational constants, dipole moment orientation, and the amine hydrogen atom positions have been used to identify the conformation of the attached chiral group from among nine possible isomeric forms. The S 1 rotational constants and electronic transition moment orientation have been obtained from high resolution molecular beam data of the band origin at 31771.56(2) cm −1. Excited state “gas phase” predictions from ab initio theory at the CIS/6-31G(d,p) and CIS/6-311 + G(d) levels are compared with the observed S 1 results and with circular dichroism (CD) data obtained for solution phase samples of ( S)-NEA in cyclohexane. Distinguishing spectral features are found in the calculated CD spectra of the three lowest energy conformers arising from rotation about the C–CH(NH 2)CH 3 bond, all of which are thermally populated at room temperature. While the predictions at both levels are in fair agreement with the observed gas phase results, CIS/6-31G(d,p) theory is found to be inadequate to model the condensed phase CD spectrum. In constrast, the calculated spectrum of the equilibrium mixture of conformers at the CIS/6-311 + G(d) level is in good qualitative agreement with the observed CD results and indicates the importance of diffuse functions for the accurate prediction of chiroptical properties of NEA. The most noteworthy exception is the predicted rotatory strength of the S 1 origin, which is overestimated by more than 10-fold. Possible reasons for this discrepancy with experiment include a reversal of S 1(L b) and S 2(L a) states from the absence of electron correlation and/or the neglect of room-temperature solvent effects. These results provide for rigorous benchmark tests of theoretical models and further elucidate the importance of the conformational structure for determinations of absolute stereochemistry from CD spectra.

Unexpected simplicity in the S1–S dispersed fluorescence spectra of C213H2

We have recorded dispersed fluorescence (DF) spectra (18 cm−1 resolution) from the JKaKc=110 rotational level of six vibrational levels in the S1 (Ã 1Au) state of C213H2. Improvements in our methods of recording and calibrating DF spectra, that have enhanced the quality of our data sets, are briefly discussed. More than 50 fractionated bright state patterns associated with approximately conserved polyad quantum numbers have been extracted from our DF data sets using a spectroscopic pattern recognition technique, extended cross correlation. These polyads extend to internal energies as high as 20 000 cm−1 above the zero-point vibrational level. The polyad fractionation patterns observed at high energy are surprisingly simple relative to the corresponding patterns of C212H2. Comparison between the DF spectra of C213H2 and C212H2 reveals slower intramolecular vibration redistribution in C213H2, particularly with nonzero quanta of CC stretch excitation. More than 15 patterns were extracted above the energy at which acetylene↔vinylidene isomerization is predicted to be energetically feasible (∼15 200 cm−1) and the dynamical information encoded in these patterns is addressed. In particular, we have analyzed a subset of the C213H2 polyads, the pure bending polyads, those with zero quanta of excitation in each of the stretch modes, (Ns=v1+v2+v3=0). The observed pure bending levels are reproduced to a root-mean-square error of <1.5 cm−1 by two different effective Hamiltonian models: an 11 parameter normal-mode and a 13 parameter local-mode model.

Visible Wavelength Electronic Spectroscopy of the DCBr ÖX̃ Vibronic Transition

The laser excitation and dispersed fluorescence spectra of the DCBr ÖX̃ vibronic bands between 570 and 600 nm were successfully obtained in a direct current (DC) discharge supersonic free jet expansion. The analysis of the dispersed fluorescence spectra results in information on the vibrational structure of the DCBr X̃1A′ state for the first time. Perturbations due to the background levels of the ã3A″ state can be recognized. The data are consistent with the previously reported triplet-singlet energy gap (the zero-point level energy gap) of 5.73±0.02 kcal/mol for HCBr. We have determined the vibrational frequencies to be 832 cm−1 (bend) and 663 cm−1 (C–Br stretch) for the X̃1A′ state. In addition to the X̃ state vibrational structure, the Ã(0,6,2) and Ã(0,5,0) levels were also observed for the first time.

Spectroscopy and dynamics of I2(B)–Ne

The I2–Ne complex has been examined using double resonance and fluorescence depletion techniques. Action spectra for I2(B,v)–Ne, detected by monitoring the I2(B,v−1) predissociation fragments, show that the Δν=−1 predissociation channel is less efficient for levels above v=32 (with no excitation of the intermolecular vibrations), and closed for v>36. From these data we obtained a revised estimate for the dissociation energy for I2(B)–Ne of D0=57.6±1.0 cm−1. Action spectra for I2(B,v=34)–Ne, detected by monitoring I2(B,v=33) fragments, revealed a progression of intermolecular vibrational levels that had not been observed previously. These levels have been assigned to T-shaped, linear, and delocalized states of I2(B,v=34)–Ne. Assignments were based on characteristic vibrational distributions exhibited by the I2(B,v−Δv) predissociation fragments. Fluorescence depletion measurements show that all of the bands in the action spectrum originate from a common ground state level. Furthermore, the one atom cage effect fluorescence from I2(B)–Ne can be depleted by transitions from the zero-point level of I2(X)–Ne. These observations indicate that the ground state wave function is delocalized, sampling both the T-shaped and linear configurations of the complex.

Laser excitation and dispersed fluorescence spectra of the HCBr ÖX̃ vibronic transition

We recorded the laser-induced fluorescence (LIF) excitation spectra of several HCBr ÖX̃ vibronic transitions between 580 nm and 630 nm in a direct current (DC) discharge supersonic free jet expansion. Dispersed fluorescence spectra, following excitation of these HCBr ÖX̃ vibronic transitions were successfully obtained for the first time. The analysis of the dispersed fluorescence spectra reveals the details of the vibrational structure of the X̃ 1A′ state as well as that of the ã 3A″ state. The vibrational frequencies were determined as 1119 cm−1 (bend), 676 cm−1 (C–Br stretch) for the X̃ 1A′ state and 994 cm−1 (bend), 733 cm−1 (C–Br stretch) for the ã 3A″ state. In addition, based upon the observation of the triplet state levels, the value of the triplet–singlet energy gap (the zero-point level energy gap) could be extracted from the dispersed fluorescence spectra and is to be approximately 2006±8 cm−1 (5.73±0.02 kcal/mol).

Pre-reactive complexes in mixtures of water vapour with halogens: characterisation of H2O...ClF and H2O...F2 by a combination of rotational spectroscopy and ab initio calculations.

Complexes H2O...ClF and H2O...F2 were detected by means of their ground-state rotational spectra in mixtures of water vapour with chlorine monofluoride and difluorine, respectively. A fast-mixing nozzle was used in conjunction with a pulsed-jet, Fourier-transform microwave spectrometer to preclude the vigorous chemical reaction that these dihalogen species undergo with water. The ground-state spectra of seven isotopomers (H2 16O...35ClF, H2 16O...ClF, H2 18O...35ClF, D2 16O... 35ClF, D2 16O...37ClF, HDO...35ClF and HDO...37ClF) of the ClF complex and five isotopomers (H2O...F2, H2 18O...F2, D2O...F2, D2 18O...Fi and HDO...F2) of the F2 complex were analysed to yield rotational constants, quartic centrifugal distortion constants and nuclear hyperfine coupling constants. These spectroscopic constants were interpreted with the aid of simple models of the complexes to give effective geometries and intermolecular stretching force constants. Isotopic substitution showed that in each complex the H2O molecule acts as the electron donor and either CIF or F2 acts as the electron acceptor, with nuclei in the order H2O...ClF or H2O...F2. For H2O...ClF, the angle phi between the bisector of the HOH angle and the O...Cl internuclear line has the value 58.9(16)degrees, while the distance r(O...Cl)= 2.6081(23) A. The corresponding quantities for H2O...F2 are phi = 48.5(21)degrees and r(O...Fi) = 2.7480(27) A, where Fi indicates the inner F atom. The potential energy V(phi) as a function of the angle phi was obtained from ab initio calculations at the aug-cc-pVDZ/MP2 level of theory for each complex by carrying out geometry optimisations at fixed values of phi in the range +/-80degrees. The global minimum corresponded to a complex of Cs symmetry with a pyramidal configuration at O in each. The function V(phi) was of the double-minimum type in each case with equilibrium values phie = +/-55.8degrees and +/-40.5degrees for H2O...ClF and H2O...F2, respectively. The barrier at the planar C2v conformation was V0= 174cm(-1) for H2O...ClF and 7cm(-1) for H2O...F2. For the latter complex, the zero-point energy level lies above the top of the barrier.

Jet spectroscopy of van der Waals dimers of 1-methoxynaphthalene: A laser induced fluorescence study

Excited state dynamics following S1←S0 transition of jet-cooled van der Waals complexes of 1-methoxynaphthalene has been investigated by measuring the laser-induced fluorescence excitation, dispersed fluorescence and two-color hole-burning spectra. The results give evidence of formation of two isomeric dimers, which show only sharp absorption and emit excimer fluorescence for excitations to the origin region of the S1 state. Excimer formation in one of the dimers occurs from the zero-point level of the locally excited state. The other dimer shows vibrational barrier and the yield of the excimer emission increases as the successive higher quanta of a low-frequency intermolecular mode are excited. The barrier in the second case has been attributed to the reorganization of the dimer geometry from the locally excited state to the excimer state. The characteristic enhancement of the yield of excimer emission with increase in excitation energy indicates the involvement of a tunneling mechanism for crossing the reorganization barrier.

Influence of the flux of H 2 and D 2 molecules on kinetics of low-temperature (down to 5 K) adsorption on the W(1 1 0) surface

The molecular beam method is applied to study the kinetics of H 2 and D 2 adsorption on the W(1 1 0) surface with the substrate temperature T S ∼5 K as a function of the molecular flux F. The character of the effect of flux changes on the adsorption kinetics is essentially different in the cases of H 2 and D 2 adsorption. In the H 2 case, the increase of F from 5×10 12 to 2×10 14 molecules/cm 2 s modifies the coverage dependence of the sticking probability S( θ) both qualitatively and quantitatively, i.e., (i) the initial sticking probability S 0 increases more than by a factor of two; (ii) the saturation coverage θ S considerably increases; (iii) the monotonic (for the lowest F) dependence S( θ) transforms into a dependence with a maximum whose height grows as F increases. In the case of D 2 adsorption, the changes of F in approximately the same range produce relatively weak effect on the adsorption kinetics. In particular, both S 0 and θ S are changed insignificantly. The increase of S 0 with rising F is assumed to be caused by the interaction of H 2 molecules in the intrinsic precursor state which leads to the nucleation of the 2D condensed phase. The greater F, the higher the nucleation probability. The 2D condensation suppresses the thermodesorption from the precursor state and thus leads to the increase of S 0. The growth of θ S with increasing F is associated with the instability, for T S ∼5 K , of the physisorbed H 2 layer whose dynamically equilibrium coverage increases as F rises. The appearance of a maximum in the S( θ) dependence can be explained by greater efficiency of the energy exchange between the incident H 2 molecules and the surface under the formation of a weakly bound molecular adlayer. The revealed distinctions in the character of the molecular flux influence on the kinetics of H 2 and D 2 adsorption for T S ∼5 K are associated with differing quantum properties of these molecules, namely, by the deeper position of the zero-point vibrational level for the heavier D 2 molecule.

Mapping the OH+CO→HOCO reaction pathway through IR spectroscopy of the OH–CO reactant complex

A hydrogen-bonded complex composed of the OH and CO reactants has been identified along the OH + CO-->HOCO reaction pathway. IR action spectroscopy in the OH overtone region has been used to examine the vibrational modes of the linear OH-CO complex, including intermolecular bending modes that probe portions of the reaction path leading to HOCO. The spectroscopic measurements have accessed highly excited intermolecular levels, with energies up to 250 cm-1 above the zero-point level, which lie in close proximity to the transition state for reaction. The OH-CO binding energy, D0 < or = 430 cm-1, has also been established from the quantum state distribution of the OH fragments following vibrational predissociation of the OH-CO complex. Complementary electronic structure calculations have been performed to characterize the OH-CO and OH-OC complexes, the transition state for HOCO formation, and the direct reaction path that connects the experimentally observed OH-CO complex to the HOCO intermediate.

Fluorescence-dip IR spectra of jet-cooled benzoic acid dimer in its ground and first excited singlet states.

The IR spectra of three isotopomers of the benzoic acid dimer have been recorded under jet-cooled conditions using the double resonance method of fluorescence-dip IR spectroscopy. In so doing, the spectra are assuredly due exclusively to dimers in the ground-state zero-point level at a rotational temperature of 3-5 K. Even under these conditions, the three isotopomers have remarkably broad spectra, extending from 2600 to almost 3150 cm-1. The spectra show extensive substructure consisting of some 15-20 transitions where only a single OH stretch fundamental should appear in the harmonic limit. The comparison of the undeuterated d0-d0 dimer with the ring-deuterated d5-d5 dimer tests the effect of mixing with the C-H stretches and overtones of the C-H bends. The mixed OH/OD ring-deuterated d6-d5 dimer shifts the frequency and changes the form of the OH stretch normal mode. The analogous OH stretch IR spectrum of the d0-d0 dimer out of the S1 excited-state zero-point level has also been recorded. In this case, much of the closely-spaced substructure is not apparent. What remains is a set of three bands separated from one another by about 180 cm-1. Preliminary results of model calculations of the anharmonic coupling, responsible for the broadening and substructure, are presented. These calculations indicate that it is OH stretch-OH bend coupling, rather than coupling with the intermolecular stretch, that is responsible for much of the observed structure and breadth.

Observations of a Quantum Symmetry Restriction in the Rovibrationally Inelastic Scattering of Glyoxal†

Rovibrationally inelastic scattering of S1 trans-glyoxal (CHOCHO, 12 modes) by H2, D2, and He is explored experimentally. Explicit attention is given to a state-to-state inelastic channel that is predicted to be forbidden on account of quantum symmetry restrictions in the symmetric top approximation for glyoxal (κ ) 0.99). This channel is 00, K′ ) 0 f 71, K′ ) 0, where 00 is the S1 zero-point level, 71 is the CHO-CHO torsional fundamental, and K′ is the quantum number for angular momentum about the top axis. After S1 r S0 pumping of the initial state, inelastic scattering to the set of final states 71, K′ ) n that includes the ∆K′ ) 0 channel is monitored by dispersed fluorescence. Relative cross sections for the rovibrational transitions are obtained for inelastic scattering by H2 (Ec.m. ) 650 and 1170 cm-1), D2 (750 cm-1), and He (750 and 1220 cm-1) to final states with K′ ranging from 0 to 15. The predicted prohibition of the ∆K′ ) 0 channel does not occur, but that channel is present with its scattering cross section reduced to only about half of the value expected if no symmetry effects were present. The partial relaxation of the quantum symmetry prohibition may plausibly be attributed to the small asymmetric top character of this near symmetric top molecule.

Excitation energy transfer of a bichromophoric cross-shaped molecule investigated by polarized fluorescence spectroscopy

The excitation energy transfer (EET) of a bichromophoric cross-shaped molecule was investigated by stationary polarized fluorescence spectroscopy in the solid state. For this purpose 2,2′,7,7′-tetrakis(biphenyl-4-yl)-9,9′-spirobifluorene was embedded in a polymeric bisphenol-A-polycarbonate (PC) matrix. The dependence of the fluorescence on concentration and wavelength was determined. The role of the intermolecular and intramolecular EET is dealt with separately and discussed by means of the degree of polarization. The intermolecular excitation energy transfer is described in terms of a Förster transfer mechanism. The intramolecular transfer is prevented for the zero-point vibrational levels by the molecular cross-shaped structure, but is found for a wide range of wavelength, presumably based on vibrationally excited states.

Vibronic interaction and triplet state photophysics of phenylisatin and oxindole

Photophysical study of phenylisatin and oxindole triplet states have been made at room temperature and in different glasses at 77K. Qualitatively, in all respects the compounds have identical spectroscopic characteristics. Phosphorescence emission, excitation along with their polarization and lifetime suggest that a perturbation of the zero-point level of emitting state ( 3ππ*) by a close-lying triplet state ( 3nπ*) leads to a number of new spectral features. The experimental observations have been interpreted satisfactorily in terms of a switch ( 3ππ* state to 3nπ*) in the character of the lowest triplet states (T 1 and T 2) and also a similar switch in the character of the excited singlet states S 1 and S 2 for a change of glass matrix from MCH to ethanol. Invoking of first order and second order spin-orbit coupling explains the phosphorescence emission unambiguously.

Tunneling in jet-cooled 5-methyltropolone and 5-methyltropolone–OD. Coupling between internal rotation of methyl group and proton transfer

The coupling of two large amplitude motions, the internal rotation of the methyl group and the intramolecular proton transfer, has been investigated for jet-cooled 5-methyltropolone, 5-methyltropolone–OD, and the 5-methyltropolone–(H2O)1 1:1 hydrogen-bonded complex by measuring the fluorescence excitation, dispersed fluorescence, and hole-burning spectra in the S1–S0 region. The vibronic bands in the excitation spectrum of 5-methyltropolone consist of four components originating from the transitions between the sublevels in the S1 and S0 states. The intensity of the bands, the frequencies, and the change in the stable conformation of the methyl group upon photoexcitation have been analyzed for 5-methyltropolone–(H2O)1 by calculating the one-dimensional periodic potential function, which provides the correlation between the internal rotational levels of 5-methyltropolone–(H2O)1 and the sublevels of 5-methyltropolone. It has been shown that the electronic transitions between the sublevels within the same symmetry are allowed in 5-methyltropolone. The tunneling splitting of the zero-point level in the S1 state is 2.2 cm−1 for 5-methyltropolone. The corresponding splitting for 5-methyltropolone–OD is less than 0.5 cm−1. A drastic decrease of the tunneling splitting for 5-methyltropolone as compared to that for tropolone (19.9 cm−1) is ascribed to a strong coupling between the two large amplitude motions in the S1 state. The existence of a similar coupling has been suggested in the S0 state of 5-methyltropolone. The excitation of the sublevel in the S1 state considerably promotes proton tunneling. This effect has been explained by the delocalization of the wave function of the internal rotation of the methyl group. The two-dimensional potential energy surface along the proton transfer coordinate and the rotational angle of the methyl group has been calculated to explain the effects of the coupling on proton tunneling.

Intermolecular vibrations of naphthalene trimer by ionization-detected stimulated Raman spectroscopy

We present nonlinear Raman spectra of intermolecular vibrational transitions in four naphthalene trimer isotopomers. The spectra, measured at 0.03 cm−1 by mass-selective ionization-loss stimulated Raman spectroscopy, reveal distinctly shaped pendular band contours, which, upon comparison to simulated pendular contours, lead to ready assignments for almost all of the observed bands. The results show clearly that the trimer has C3h (or very nearly C3h) symmetry with the naphthalenes arranged such that their long axes are parallel to one another. Comparison of the experimental results to the results of calculations of naphthalene-trimer intermolecular spectra performed by assuming harmonic intermolecular modes and by taking the intermolecular potential energy surface to be a pair-wise additive one reveals significant qualitative agreement. This agreement strongly suggests a relatively rigid cluster with no large amplitude intermolecular motions within 100 cm−1 of the zero-point level. Finally, the results highlight the dominance of librational motions in giving rise to scattering intensity in the intermolecular portion of the cluster’s Raman spectrum.

Photophysics of tropolone

The excited state fluorescence lifetimes of isolated tropolone in a supersonic expansion have been measured as a function of vibrational energy in the E vib=0–293 cm −1 region. A fluorescence quantum yield of 0.06 is estimated for the zero-point level of S 1; the lifetimes decrease rapidly with increasing vibrational energy. Semi-empirical calculations of the energies of nearby excited states suggest that intersystem crossing to T 3 may dominate the non-radiative decay of vibrationless S 1, whereas the decrease of lifetime with increasing E vib is due to an increasing rate of internal conversion via the `proximity effect'.

Phonon squeezed states: quantum noise reduction in solids

This article discusses quantum fluctuation properties of a crystal lattice, and in particular, phonon squeezed states. Squeezed states of phonons allow a reduction in the quantum fluctuations of the atomic displacements to below the zero-point quantum noise level of coherent phonon states. Here we discuss our studies of both continuous-wave and impulsive second-order Raman scattering mechanisms. The later approach was used to experimentally suppress (by one part in a million) fluctuations in phonons. We calculate the expectation values and fluctuations of both the atomic displacement and the lattice amplitude operators, as well as the effects of the phonon squeezed states on macroscopically measurable quantities, such as changes in the dielectric constant. These results are compared with recent experiments. Further information, including preprints and animations, are available in http://www-personal.engin.umich.edu/∼nori/squeezed.html.

State-specific vibrational anharmonicities in cyclobutadiene and evidence for fast automerization by C412H4

MO computations of vibrational spectra for 1,3-cyclobutadiene (CB) and CB:CO2 van der Waals molecule isotopomers at the MP2/6-311++G(2d,2p) level of MO theory are analyzed together with the extensive Ar/CB matrix-isolation spectra of the Michl group, the low 3.2 kcal/mol (ZPE-corrected) MO-computed energy of the square-planar saddle-point for CB reported by Nakamura et al., and opportune one-dimensional analysis of the automerization energy levels of C412H4. The composite interpretation results in the assignment of the strongly anharmonic Raman transitions of Ar/CB observed at 723 and 1678 cm−1 to the E2-E0 and E4-E0 transitions, respectively, in the automerization coordinate. The C13 and D isotope dependencies of the nominal C=C stretching and δCH wagging vibrational normal modes of the CB valence isomers depend markedly on whether the labeled HC=CH moieties of CB are mass equivalent or not, and the pronounced normal mode differences are viewed as providing different gateways into the global anharmonic PES domains. The automerization coordinate Qa originates as the nominal C=C stretching normal coordinate of ag symmetry, followed by a curving to more efficiently interchange the C=C and C–C bond lengths as the square-planar D4h configuration is approached. The latter is crossed at a geometry dilated with respect to the saddle-point, thereby providing a barrier configuration about 5 kcal/mol above the PES minima (i.e., a dilation energy of about 2 kcal/mol plus the 3.2 kcal/mol saddle-point energy). The computed zero-point levels of C412H4 (1-D modeling) are separated by E1-E0=11 cm−1 to suggest ZP tunneling rates reaching the picosecond time scale. The analysis exposes new aspects of the intramolecular dynamics of CB, and the proposed automerization mechanism accounts for all of the presently known information pertaining to this interesting property of the cyclobutadiene molecule.

Emission spectroscopy of jet-cooled CS2 upon excitation of the Σg+1→1B2(1Σu+) transition in the 48 500–51 000 cm−1 region

The jet-cooled emission spectroscopy of gaseous carbon disulfide (CS2) was performed in the 48 500–51 000 cm−1 region in order to investigate changes in the early time dynamics of CS2 in the predissociative B21 (1Σu+) state. The spectra show emission into progressions of both the symmetric stretching (ν1) and bending (ν2) motions and combinations bands of these two motions. Analysis shows that the contribution of symmetric stretching and bending motion varies over the range of excitation wavelengths and the progression of ν2 decreases in intensity relative to the progression in ν1 as one scans through the energetic region near the barrier to linearity of this bent excited state molecule. We estimate the barrier height to be approximately 3400 cm−1 above the zero-point level of the B21 (1Σu+) state.

Electronic spectroscopy and ab initio quantum chemical study of the Ã(1A″)−X̃(1A′) transition of CFBr

The Ã(1A″)−X̃(1A′) electronic transition of jet-cooled CFBr has been investigated by spectroscopic and ab initio theoretical methods. Laser induced fluorescence (LIF) excitation spectroscopy was used to explore the rovibronic levels in the à state, and dispersed fluorescence spectroscopy was used to study ground-state vibrations. Analysis of these spectra yielded gas-phase vibrational frequencies and anharmonicity constants in both electronic states. The computed ab initio vibrational frequencies in both X̃ and à states are in good accord with the experimental values. The Ã-state fluorescence lifetimes varied between 100 ns and 3 μs as a function of excited vibronic state. The highest lying levels displayed a shortened fluorescence lifetime, and some vibronic states that involved ν1 (the CF stretch) exhibited shortened lifetimes (300–500 ns) irrespective of the vibrational energy. Vibronic structure in the LIF spectrum disappeared for vibrational energy in excess of 2957 cm−1. Calculations of the Ã-state potential-energy surface show that it has a small barrier to dissociation to CF+Br with a barrier height in good accord with observed termination of fluorescence. The predicted photochemical pathway to production of CF+Br fragments was proven experimentally by detection of CF fragments. The photofragment excitation spectrum showed strong, increasingly broad vibronic structure at higher energies than the LIF spectrum. At lower energy, sharp but weaker vibronic structure was still evident, overlapping the LIF spectrum. There appears to be two photochemical mechanisms to produce CF+Br, one direct and one indirect. We estimate the height of the barrier to direct dissociation to lie 3250±150 cm−1 above the zero-point level of the à state. The asymptotic thermochemical dissociation limit is estimated to lie ⩾1100 cm−1 lower. The thermochemical bond dissociation energy for the C–Br bond in CFBr was thereby estimated to be Ediss⩽23 180 cm−1, which led to an estimate of the heat of formation for CFBr, ΔfH2980⩾86 kJ mol−1.