The heat of formation (AH¡(29& K)) of HNCO is determined to be -24.9J2°87 kcal/mol (based on Afff(NH) = 85.2 kcal/mol). This value is obtained by measuring the threshold for the production of NH(a'A) and by determining the energy contents of the NH fragment and the CO cofragment produced by photolysis of HNCO it wavelengths near the threshold. Saturated laser-induced fluorescence is used to determine the internal state distribution of NH(a'A), and multiphoton ionization is used to measure the internal state distribution of CO. An upper limit for the branching ratio of NCO/NH production from photodissociation of HNCO at 193 nm is determined from an analysis of kinetic experiments to be 0.10. To clarify the mechanism of photodissociation, HNCO fluorescence-excitation and NH(a'A) action spectra are also measured. They imply that two excited states of HNCO are present where only one had previously been considered.
The fluorescence spectrum, one photon fluorescence excitation spectrum, and single vibrational level decay kinetics have been measured for 1,6-diphenyl-1,3,5-hexatriene seeded in helium in a supersonic free jet. The 2 1Ag doubly excited state (S1) is located more than 3400 cm−1 below the strongly absorbing 1 1Bu state (S2) which is 29 157 cm−1 above the ground state. The lowest energy observed vibronic band of S1 is at 25 741.8 cm−1; emission produced by excitation into this band has a lifetime of 90.7 ns. The vibronic development of the excitation spectrum shows that this band is a vibronically promoted false origin. Fluorescence excitation intensity for this band is smaller than that of the S0 to S2 fluorescence excitation origin by at least a factor of 104. Direct excitation of S2 in the isolated molcule is followed by relaxation, fast on the nanosecond time scale, to isoenergetic vibrationally excited levels of S1. This is apparent both from the observed fluorescence spectrum and the fluorescence decay time (48.1 ns at the S2 origin) which is more than an order of magnitude longer than the intrinsic fluorescence lifetime of S2.
Vibrational state controlled bond cleavage in the photodissociation of isocyanic acid (HNCO) J. Chem. Phys. 102, 8440 (1995); 10.1063/1.468835 CO product distributions from the visible photodissociation of HCO J. Chem. Phys. 97, 9036 (1992); 10.1063/1.463330 Photodissociation dynamics of Mo(CO)6 at 266 and 355 nm: CO photofragment kineticenergy and internal state distributions J. Chem. Phys. 94, 7937 (1991); 10.1063/1.460128 Rotational state distributions of NH(a 1Δ) from HNCO photodissociationThe internal state distributions of CO produced by photodissociation ofHNCO at 1930 (230.1 nm) and at 10 200 cm-1 (193.3 nm) in excess ofthe dissociation energy are determined from multiphoton ionization spectra of the CO fragment measured under collision-free conditions. The rotat~onal state distribution of the CO produced at the lower photolysis energy is charactenzed by a temperature of (491 ± 23) K. The rotational state distribution of CO produced by photodissociation at the higher photon energy in not well characterized by a temperature. This latter distribution has maximum popUlation near J = 37, extends beyond J = 65, and accounts for -20% of the available energy in excess of that necessary to rupture the HN-CO bond. An impulsive dissociation model assuming that dissociation occurs from an excited ~NCO co~p~ex con.taining a nonlinear N-C-O configuration accounts for the average CO rotatIonal eXCItatIon whIle phase-space theory does not agree with the observed distributions. Fitting a semiclassical model to the data using a logically constructed potential energy surface and a ground-state-dependent function provides a useful parametrization for the impulsive dissociation. Although not absolute, this analysis suggests that the dissociation occurs directly on a repulsive excited state potential energy surface.1568
Highly resolved emission and one-photon fluorescence excitation spectra for 1,4-diphenyl-1,3-butadiene seeded in a supersonic expansion of helium have been measured. The spectra show a long-lived (52.8 nsec for excitation at the 0-0) state at 29,652.5 cm-, approximately 1,150 cm-' below the well-characterized 'Bu state, which is assigned as 'Ag-i.e., we have directly observed a polyene 1Ag state in the gas phase. Emission spectra and decay times for the 1Ag state were measured at a number of different excitation energies. These data clarify the ordering of excited singlet states and the photophysical behavior of diphenylbutadiene.The connection between electronic structure and photochemical behavior in the linear polyenes is the focus of a number of spectroscopic and theoretical investigations (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14). Particularly for the shorter polyenes, the photophysics of these molecules is dramatically affected by the close proximity of the two lowexcited singlet states (21Ag and 11BJ). In fact there is evidence that the separation and even the ordering of these states is a strong function of effective conjugation length and molecular environment (8,12,(15)(16)(17). While high-resolution spectroscopy in condensed and gas phases has greatly increased our insight into the photochemical behavior of polyenes, in particular octatetraene (8, 9), less is known about the low-lying singlet states of hexatriene and butadiene. This is primarily because of the lack of detectable fluorescence (8), which is not a problem for the a,w-diphenylpolyenes.Despite the fact that 1,4-diphenyl-1,3-butadiene fluoresces with reasonable yield, there are still some outstanding questions regarding the proper description of the low-lying singlet states of this molecule. On the basis of one-and two-photon spectra measured for diphenylbutadiene in EPA at 77 K, the 21Ag state has been located at 27,900 cm-1, 130 cm'-below the origin of the l1B1h state (1). However, the two-photon fluorescence-excitation spectrum of diphenylbutadiene in low-temperature hydrocarbon glasses and solutions can be interpreted as showing an 'Ag state higher in energy than the 1B, (11,14). This discrepancy is not explained by simple solvent-shift theories even though these theories quantitatively account for the solvent dependence of the ground state-to-l1Bu transition energy (12). The measured lifetime of the emitting state of diphenylbutadiene in cyclohexane at room temperature is 1.8 nsec, which is consistent with either state ordering (10). Molecular orbital calculations using configuration interaction through double excitations predict at least two low-lying 1Ag states; one at 28,000 cm'1, which has roughly 20% polyene character, and one at 30,440 cm-', which has roughly 45% polyene character (1). Thus, not only is the ordering of the 'Ag and 1Bu states at issue, but exactly which 1A state is lowest in energy is also open to question. Bennett and Birge (1) have assigned the state observed in their two-photon spectrum as a trans...
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