U. Brühlmann

Sub‐Doppler laser‐induced fluorescence measurements of the velocity distribution and rotational alignment of NO photofragments

The photodissociation of dimethylnitrosamine, (CH3)2N–NO, at 363.5 nm produces ro‐vibrationally excited NO fragments. With two‐photon laser‐induced fluorescence the Doppler profiles of single rotational transitions were probed, using various polarization schemes for dissociation and probe lasers. These measurements provided a detailed product state distribution, the three‐dimensional recoil velocity distribution of specific fragment states, and the alignment of their angular momentum. We present evidence of the presence of correlations between fragment recoil direction and alignment of fragment angular momentum.

Photodissociation of methylnitrite: State distributions, recoil velocity distribution, and alignment effects of the NO(X 2Π) photofragment

The photodissociation of CH3ONO→CH3O+NO at 363.9 nm generates vibrationally, rotationally, and translationally excited fragments (fNOvib =3%, fNOrot =15%, fNOtrans =33%). By means of two‐photon LIF and sub‐Doppler spectroscopy in combination with various polarization schemes of dissociation and probe lasers, the nascent NO(X 2Π) photofragment was characterized with respect to state distributions and three‐dimensional recoil velocity distribution. Furthermore, the rotational alignment and the Λ‐state populations were determined. Through the rotational alignment dependence of Doppler profiles probed by differently polarized transitions (R and S), the J–v vector correlation was assessed. Based on these results, stereochemical and dynamical information about the dissociation was obtained which shows that the fragmentation process is planar and takes place within 210 fs.

Spin-state (Π12, Π32) population of no (X, ν″=0, 1, 2, 3) fragments after state-selective photodissociation of CH3ONO

Abstract Following selective excitation of CH3ONO into the NO stretching mode (ν′=0, 1, 2, 3 ) of the S1 state, the photodissociation products NO(X; ν″=0, 1, 2, 3) were analysed by measuring the rotational energy distributions in the four sublevels 2Π± 1 2 , 2Π± 3 2 . Depending on the quantum numbers ν′ (parent) and ν″ (fragment) of the state-to-state process, the spin states Π 1 2 and Π 3 2 are found either equally or differently populated. This effect is proposed to be the result of an adiabatic and a non-adiabatic dissociation path with respect to the vibronic potential surfaces.

Triplet state quenching by ground state molecules of the same kind. Xanthione in solution

Abstract The highly efficient quenching of thione triplet states by ground state molecules of the same kind was investigated for xanthione by means of phosphorescence lifetime measurements in a variety of solvents, in some cases as a function of temperature. The quantum yields of photodecomposition of xanthione were also determined. It is shown that the triplet quenching process is diffusion-controlled and physical in nature. The short-range quenching mechanism is suggested to involve triplet excimer formation with subsequent radiationless decay.

Photodissociation of dimethylnitrosamine

Abstract Photodissociation of (CH 3 ) 2 N-NO following S 1 (nπ * ) ← S 0 excitation yields (CH 3 ) 2 N − and NO with a quantum yield of 1.03 ± 0.10. These fragments recombine leaving no stable photopioducts. A fraction of NO produced by photolysis is vibrationally excited. The rate of the NO( v = 1) relaxation in collision with (CH 3 ) 2 N-NO, measured by IR fluorescence, is (1.47 ± 0.03) × 10 4 s −1 Torr −1 .

NO(X 2Π) rotational distributions from the photodissociation of NOCl and NOBr at 450 and 470 nm

Abstract The rotational, Λ doublet and spin state distributions of the NO fragment following dissociation from NOCl and NOBr have been measured using a one-colour photolysis analysis laser scheme. Dissociation was by one photon at 450 or 470 nm while two-photonnon-resonant LIF at the same wavelength was used to probe the NO fragment. The NO rotational distributions for both precursors are found to be inverted and non-Boltzmann with states up to N=60 populated. Similar Λ doublet ratios of 1.7 and 1.6 for NOCl and NOBr respectively are observed. A statistical F2/F1 spin state population has been measured for NO from NOBr photolysis, however, the spin ratio from NOCl is found to be non-statistical with F2/F1 = 2.1. Sub-Doppler nascent NO line profiles reveal Cl is formed principally in the lower P 3 2 state but Br in either P 1 2 or P 3 2 depending on the vibrational and rotational state of the NO fragment. In addition, the line profiles demonstrate a predictable perpendicular v-j correlation corresponding to planar dissociation and also allow the assignment of the accessed electronic transitions. Coupled with alignment measurements, A 0(2) and βeff, the excited state symmetries are assigned.

Photofragmentation LIF spectroscopy of NOCl at dissociation wavelengths > 450 nm. Parent electronic spectrum and spin state and Λ-doublet populations of nascent NO and Cl fragments

Abstract The primary photodissociation of nitrosyl chloride, NOCl + hv →NO+Cl, has been investigated at selected excitation wavelengths > 450 nm. By means of a one-colour (1 + 2)-photon experiment, i.e. one-photon absorption for parent dissociation and two-photon absorption for LIF to probe NO, the vibrational and rotational state distributions of nascent NO(X 2 Π) were measured and in combination with polarized Doppler spectroscopy also the translational energy obtained. Based on the anisotropy parameter β the visible and near-UV part of the electronic absorption spectrum of NOCl was assigned. After excitation at 475 nm the energy partitioning of both, the NO and Cl fragments, reveals that the dissociation proceeds in the molecular plane of the parent preferring (≈ 65% efficiency) the channels which lead to NO( 2 Π − ) and Cl( 2 P 3 2 ). The NO Λ-doublet and spin-orbit state population ratios 2 Π −3 2 : 2 Π +3 2 : 2 Π −1 2 : 2 Π +1 2 were found to be 4.9:3.6:3:1.

Vector correlations from doppler–broadened lineshapes. State-selected dissociation of methyl nitrite

The theoretical framework needed to analyse sub-Doppler lineshapes obtained by 2 + 1 LIF spectroscopy is outlined and applied to data for the state-selected photodissociation of cis-methyl nitrite (CH3ONO). Excitation into the second overtone of the terminal NO stretch (ν′= 2) produces NO fragments with zero, one or two vibrational quanta and a highly correlated set of vector properties. These vector properties can be quantified in terms of five bipolar moments, which include the rotational alignment, the translational anisotropy parameter, the second Legendre moment of the (v, j) correlation and tow moments which characterise the three-vector (µ,ν,j) correlation [β20(22) and β20(24)]. In principle, these bipolar moments can be used to separate the dissociation dynamics into the influence of a dissociation lifetime and a dissociation geometry. The experimental evidence for the photolysis of methyl nitrite implies an essentially planar dissociation geometry leading to a strong perpendicular (ν, j) correlation [β00(22)=–0.34 ± 0.05]. The slight deviation from complete planarity is probably caused by torsional forces. The dissociation lifetime derived from the value of the translational anisotropy parameter appears to be a function of the fragment vibrational level. A simple model which explains many of the observations is presented and discussed in the light of preliminary classical trajectory calculations.

Photodissociation of CH3ONO by a direct and indirect mechanism

Abstract The S 2 potential-energy surface of CH 3 ONO has been calculated for the coordinates R NO and R ON using the MCSCF ab initio method. To complete the experimental data set of the S 2 photodissociation, we have measured the vibrational- and rotational-state distributions of the NO(X 2 Π) fragments, and discussed the distinctly different product energy distributions from the direct S 2 and the indirect S 1 dissociation on the basis of the potential-energy surfaces.

Deuterium isotope effect in the T1 → S0 radiationless transition of propynal in the gas phase

Abstract The phosphorescence lifetimes of propynal- h 1 and propynal- d 1 have been measured at room temperature in the 40 mTorr-1 Torr pressure range The reciprocal of the zero-pressure lifetime ( k 0 ) is (3.10 ± 0.05) × 10 3 and (1.70 ± 0.04) × 10 3 s −1 for propynal- h 1 and propynal- d 1 For both compounds the rate constant for self-quenching between triplet and ground-state molecules is k SQ = (1 2±007) × 10 3 Torr −1 s −1 The deuterium isotope effect is attributed to the T 1 → S 0 radiationlcss decay, for which k H ISC / k D ISC = 2 4