The electronic spectrum of NOCl: Photofragment spectroscopy, vector correlations, and a b i n i t i o calculations

Abstract

The electronic absorption spectrum of NOCl in the region 620–180 nm is assigned by using vector properties of the NO photofragment and the results of ab initio calculations at the CI level. In assigning the electronic spectrum, we take into account the recoil anisotropy, rotational alignment, and Λ-doublet populations of NO, as well as the calculated vertical excitation energies, oscillator strengths, and the nature of the orbitals involved in the transitions. In the experiments, we use expansion-cooled samples and measure the recoil anisotropy parameters from the Doppler profiles of selected NO A 2Σ+←X 2Π rotational lines. The alignment parameters and Λ-doublet populations are derived from the rotational spectra using different laser polarizations and excitation–detection geometries. The theoretical calculations treat all low-lying singlet and triplet states. The calculations yield least energy paths for the excited states, with optimized rNO and ClNO angle as a function of rClN, as well as the angular dependences of the potentials and oscillator strengths of the singlet–singlet transitions. The following assignments are proposed for the main absorption bands: (1) E band—T1(1 3A″)←S0(1 1A′); the transition borrows intensity by mixing with remote singlet states, predominantly the 4 1A′ state; (2) D and C bands—S1(1 1A″)←S0(1 1A′); the C band corresponds to excitation of ν′1 in S1; (3) B band—S3(2 1A′)←S0(1 1A′); (4) A band—S5(4 1A′)→S0(1 1A′). The assignments proposed here are in full agreement with all the experimental observations and the results of the calculations. Despite the shallow minima calculated for the T1 and S1 surfaces, dissociation on all the surfaces is fast, and the implications of the results to the dissociation dynamics are discussed.

Femtosecond Pump−Probe Studies of Nitrosyl Chloride Photochemistry in Solution

“…As mentioned in the Introduction, photochemical studies of gaseous ClNO employing wavelengths resonant with the A-band have shown that Cl and NO formation is the dominant photochemical pathway. 25,36,[39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56] In contrast, the structural isomer (ClON) is the only photoproduct observed in low-temperature matrix studies. 63,64 The results presented here represent the first detailed measurements of ClNO photochemistry in solution.…”

“…38 The photochemistry of gaseous ClNO is dominated by formation of the photoproducts, NO ( 2 Π ( 1 / 2 , ( 3 / 2 ) and Cl ( 2 P 1/2,3/2 ), with near-unity quantum yield. 25,36,[39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56] The absorption spectrum of ClNO consists of a series of bands referred to as the K through A bands as one proceeds from low to high energy. 57,58 The A-band dominates the absorption spectrum and has an absorption maximum of 196 nm in the gas phase.…”

“…52,54,55 This hypothesis is supported by theoretical studies of ClNO. 45,[59][60][61] However, UV resonance Raman intensity analysis studies of gaseous ClNO could be explained in terms of a single excited state. 56 There are only a handful of studies regarding the photochemistry of ClNO in condensed environments.…”

Photolysis of ClNO adsorbed on MgO(100)

“…ClNO has a low bond dissociation energy (13 000 cm-l) (29) and absorbs light throughout the visible and ultraviolet -spectral regions that are easily accessed with tunable dye lasers. The 300 K absorption spectrum consists of several broad absorption bands (32), which have recently been assigned (30). Its NO product state distributions, photofragment yield spectra, recoil anisotropy, rotational alignment, NO A-doublet populations and the nature of the orbitals involved in electronic transitions have been determined both experimentally and theoretically (28-3 1).…”

“…At 355 nm, gas phase ClNO is excited via a parallel transition, s3(21Ar) t sO(llA'), yielding rotationally hot NO, i.e., = 46.5 (29,30). Almost all of the NO is in v" = 0.…”