Photofragment Yield Spectra Research Articles

Photodissociation dynamics of jet-cooled ClNO on S1(1 1A″): An experimental study

We report measurements of photofragment yield (PHOFRY) spectra and NO E, V, R distributions following dissociation of jet-cooled ClNO on the S1(1 1A″) electronic surface. The dissociative S1(1 1A″)←S0(1 1A′) transition shows diffuse vibrational structure with a progression in ν1, the NO stretch. The absorption and PHOFRY spectra consist of two bands, corresponding to excitations into S1(000) and S1(100), whose widths are 1300±100 and 1000±70 cm−1, respectively. The relative partial absorption cross sections are S1(000):S1(100)=2.3:1.0. The narrowing of the absorption bands with increasing ν1 quanta is a consequence of the mismatch between ν1 and the free NO vibrational frequency. Dissociations on S1(000) and S1(100) yield NO in v″=0 and 1, respectively. The NO(X2∏) rotational distributions in v″=0 and 1 are inverted, peaking at J″∼30.5 with widths of 10±1 J″, and they do not vary significantly when the photolysis laser is scanned across the absorption band. The evolution of NO vibrational and rotational excitations appear to be largely uncoupled. In NO v″=0 and 1, the upper spin–orbit state 2∏3/2 is more populated than the lower state 2∏1/2. For both v″=0 and 1, the Λ-doublet ∏(A″) component of NO(2∏1/2) is more populated than the ∏(A′) component by a ratio of ∼3:1, as expected for excitation to a π* orbital of a″ symmetry, but this propensity is much lower for NO(2∏3/2), possibly due to perturbations with another surface. The absorption spectra and NO V, R distributions are in good agreement with recent dynamical calculations on a three-dimensional (3-D) potential-energy surface (PES) calculated ab initio. The vibrational distribution appears to be determined near the Franck–Condon (FC) region, while final-state interactions affect the rotational distributions at larger Cl–NO separations.

The influence of excited-state vibrations on fragment state distributions: The photodissociation of NOCl on T1(1 3A″)

NO V,R distributions are reported following photodissociation of jet-cooled NOCl from selected vibrational levels of the T1(1 3A″) state. By varying the photolysis wavelength while monitoring selected rovibrational levels of NO, a photofragment yield spectrum showing the same diffuse vibrational structure as the absorption spectrum is obtained. NO rotational distributions are obtained at photolysis wavelengths corresponding to peaks in the absorption spectrum. We find that the NO stretching vibrations of T1 evolve adiabatically into NO vibrational excitations [i.e., excitation of v1 quanta of the T1 NO stretch yields predominantly NO(v″=v1) ]. The NO rotational distributions depend only on the number of T1 bending quanta, v3. The shapes of the distributions reflect the number of nodes in the bending wave functions, and similar rotational distributions are obtained following excitation of vibronic levels with equal number of ν3 quanta, but different number of ν1 quanta (v1=0–2). We also find that the excited NO(2Π3/2) state is much more populated than the lower NO(2Π1/2) state, and the widths of the absorption features increase with increasing number of ν3 quanta, but decrease with increasing ν1 excitation. The results are explained in terms of a model in which the bending and NO stretch motions in the excited state are largely uncoupled. In order to explain the multimodal rotational distributions, we calculate separately the components of the NO rotational excitation that derive from the angular momentum inherent in the bending wave function and the angular anisotropy in the potential-energy surface. We find, using the momentum representation of the harmonic oscillator, that the rotational distributions map the bending wave functions and exhibit minima for v3>0. In addition, a unidirectional torque generated by the angular anisotropy in the potential causes shifts in the distributions calculated by the pure Franck–Condon model that are bending level dependent. The observation that the absorption linewidth decreases with increasing NO stretch excitation in the parent T1 state may be a manifestation of the energy mismatch between the frequency of the NO stretch in NOCl and free NO; the increased mismatch with increasing stretch quantum number results in slower dissociation.

Free-jet studies of the visible spectroscopy of some perhalonitrosomethanes

Recent work on the electronic spectroscopy of halogen substituted nitrosomethanes in the visible spectral region is reviewed with particular reference to laser-induced fluorescence and photofragment yield spectra in supersonic free-jet expansions. The A( n, π*)- X electronic spectra in the 650–720 nm region are dominated by progressions in low frequency torsional modes, that reflect eclipsed to staggered dihedral conformational changes upon electronic excitation. The observed torsional progressions can be understood well on the basis of spectral simulations that utilize a simple one-dimensional Hamiltonian for the hindered internal rotation. The relevance of these laser spectroscopic studies for current work on the state-selected photophysics and photodissociation dynamics of the title compounds is highlighted.

State-selective photochemistry: the predissociation dynamics of jet-cooled dichlorofluoronitrosomethane (CCl2FNO) in the 600 nm region

The predissociation dynamics of the substituted nitrosomethane CCl2FNO (dichlorofluoronitrosomethane) in the 660 nm region have been studied using jet-cooling, state-selective excitation by a pulsed dye laser and delayed LIF probing of nascent NO(X). Energy disposal into NO and the shapes of its rotational state distributions are relatively insensitive to the selected parent molecular vibronic state but mimic statistical (prior) distributions, providing little evidence for state-specific effects. In contrast, photofragment yield spectra and fluorescence decay profiles of the parent molecule indicate that skeletal rocking and bending modes in combination with the torsion (a″ symmetry) act as promoting modes for the radiationless transition, resulting in the dissociation of the weak C—N bond. The spin–orbit ratios of the NO fragment appear to be controlled by the topology, i.e. the details of the potential-energy surfaces involved. Intersystem crossing to the T1(n,π*) state is found to affect the dynamics of all but the lowest S1(n,π*) levels.

The spectroscopy, photophysics and photodissociation dynamics of jet-cooled CF3NO[Ã(n,π*)

The LIF excitation spectrum of jet-cooled CF3NO in the visible has been reinterpreted and the torsional structure has been analysed in detail yielding a V′3 barrier to internal rotation of 601.5 cm–1. The predissociation dynamics of jet-cooled CF3NO(A1A″) have been re-examined using single vibranic level excitation with a pulsed dye laser and fluorescence decay measurements. Quantum beats and biexponential fluorescence decay behaviour indicate that all levels of the S1(n,π*) state are coupled to levels of the T1 state. The NO photofragment yield spectrum indicates that vibrations with CNO bending character act as promoting modes for the radiationless transition out of S1. Rotational distributions of the nascent NO photoproduct have been characterised in detail and are found to deviate little from statistical distributions, but the F2/F1 spin-orbit branching ratio is very non-statistical, favouring the lower 2Π1/2 state of NO. The threshold for the dissociation of CF3NO(D0) and the electronic origin of the Aâ†�X transition (T00) have been redetermined and are found to coincide (D0= 13 980 ± 60 cm–1, T00= 13 929.7 cm–1). Intersystem crossing and the T1 state are found to be important in the predissociation dynamics of all but the lowest A-state levels.

Laser photofragment spectroscopy of jet-cooled CClF 2NO

Photofragment yield spectra of jet-cooled CClF 2NO in the visible region have been recorded using pulsed laser dissociation in conjunction with two-photon LIF probing of the nascent NO fragment. Photodissociation of the cold parent at 646.6 nm yields NO with a statistical rotational state distribution but a colder than statistical spin-orbit distribution. A dissociation energy of 13500 ± 350 cm −1 has been evaluated for the C-N bond of the parent nitroso compound.