D. Solgadi

State-selective photochemistry: Energy distribution in the no fragment a photodissociation of the CH3ONO nπ* state

Abstract The vibrational and rotational state distributions of the NO X 2 Π fragment resulting from photodissociation of cis-CH 3 ONO excited in specific NO stretching vibrational levels of the neπ* state at 364, 350, 338, 328 and 318 nm have been determined. The NO X ν″ = 0, 1, 2, 3, 4 probed by two-photon laser-induced fluorescence exhibits a large amount of rotational excitation which is independent of the excitation wavelength. The NO X vibrational population varies with the vibrational level prepared in the parent excited molecule and the results show that the excess vibrational energy deposited in the N0 stretching mode of the CH 3 ONO nπ* state is retained in the vibration of the free NO fragment. The rotational excitation is interpreted in terms of interfragment ccupling due to the impulse applied to the recoiling fragments. The vibrational distribution is more likely to be explained by a simple model based on intrafragment coupling which takes into account the change in N0 bond length between the ground and dissociative state of the molecule.

Rotational and electronic anisotropy in no X2Π from the photodissociation of CH3ONO

Abstract Alignment effects m the highly rotating NO X 2 Π ν″ = 1 fragment generated from photodissociation of CH 3 ONO nπ * at 355 nm have been probed by two-photon laser-induced fluorescence spectra. From polarization experiments the rotational angular momentum of NO fragments was shown to be aligned preferentially along the transition moment of the parent molecule. The populations of the A doublet and spin components are not m equilibrium and exhibit J -dependent ratio. The favored component of the A doublet corresponds to the pπ lobe of the NO fragment lying predominantly perpendicular to the plane of rotation as it is expected from the symmetry properties of NO electronic charge distribution in the parent CH 3 ONO 1 A″ state. The results are consistent with an impulsive dissociation with fragments recoiling close to the initial molecular plane.

Intracluster hydrogen transfer followed by dissociation in the phenol–(NH3)3 excited state: PhOH(S1)–(NH3)3→PhO•+(NH4)(NH3)2

The study of the phenol–(NH3)3 cluster with two-color two-photon ionization shows that the main ion observed with delays between the lasers up to a few hundred nanoseconds is the (NH4)+(NH3)2 fragment, resulting from direct ionization of the (NH4)(NH3)2 product coming from the reaction: PhOH(S1)–(NH3)3→PhO•+(NH4)(NH3)2.

Experimental study of the cold mercury dimer

The mercury dimer is among the most weakly bound metal dimers and has been extensively studied.1 The ground state O+g dissociation energy has been considered to lie between 0.55 eV (440 cm−1) and 0.091 eV (730 cm−1). We report here a spectroscopic study of Hg2 in a supersonic jet. The first optical transition, 1u←O+g, was characterized by its fluorescence excitation spectrum and the binding energy of the ground state has been measured precisely through the threshold of collision induced dissociation of Hg2 1u to Hg(1S0)+Hg(3P0).The mercury dimer is among the most weakly bound metal dimers and has been extensively studied.1 The ground state O+g dissociation energy has been considered to lie between 0.55 eV (440 cm−1) and 0.091 eV (730 cm−1). We report here a spectroscopic study of Hg2 in a supersonic jet. The first optical transition, 1u←O+g, was characterized by its fluorescence excitation spectrum and the binding energy of the ground state has been measured precisely through the threshold of collision induced dissociation of Hg2 1u to Hg(1S0)+Hg(3P0).

Photofragmentation of CH3ONO at 355 nm. Energy distribution in the no fragment

Abstract The 355 nm photodissociation of gaseous methyl nitrite has been studied by monitoring the nascent NO product using a two-photon laser-induced fluorescence technique. The nascent vibrational and rotational distributions have been measured: the NO fragment is produced in four vibrational levels of the ground state (ν″ = 0, 1, 2, 3) and the distribution peaks at ν″ = 1; all these vibrational states are rotationally excited. The NO translational motion has been evaluated by recording the linewidth of individual rotational lines. The translational energy represents 36% of the available energy ( E av1 = 14320 cm −1 ) and the internal energy of the NO fragment is ≈26% of E av1 .

A time-resolved photoelectron study of the double excited-state proton-transfer reaction in 7-azaindole dimer

Abstract A study employing picosecond and subpicosecond excitation in a mass and photoelectron spectrometer is reported for the 7-azaindole (7-AI) dimers, reactive and unreactive. The 7-AI photoelectron spectrum is structured and has a sharp ionisation threshold at 8.17 eV above the neutral ground electronic state. The reactive dimer ionisation threshold was measured as 7.19 eV. The excited-state lifetime of the reactive dimer was measured by a technique that monitors the ionisation signal as a function of pulse duration. 1 + 1 resonance ionisation photoelectron spectra were recorded using 0.8 ps and 5 ps pulses. Our results indicate a lifetime substantially less than a picosecond, however, consistent with recent real time studies. Advantages of the method are discussed.

OBSERVATION OF AN INDIRECT PATHWAY IN THE FEMTOSECOND STUDY OF ALKYL NITRITE PHOTODISSOCIATION IN THE S1 STATE

The present work has addressed the question of the dissociation time of four alkyl nitrites upon photoexcitation in the S1 state (methyl nitrite, n‐ , and t‐butyl nitrites and i‐amyl nitrite). The time resolved Laser Induced Fluorescence technique has been used in the femtosecond regime under bulk conditions. The photodissociation has been initiated at 351 nm by 150 fs pump pulses, and has been probed using a two‐photon process at 467 nm by 200 fs pulses. The LIF signal has qualitatively the same shape for the four nitrites: it passes through a transient peak before reaching a plateau. The two‐photon process that induces the detected fluorescence is nonresonant for detection of the dissociation product NO through the A←X transition. Conversely, the two‐photon process is resonant or quasiresonant for detection of the excited nitrite molecule in the S1 level before it dissociates. This leads to an enhanced detection efficiency of the non‐dissociated excited molecule versus that of the NO fragment. A simple ...

Fluorescence excitation spectrum of the Si–Ar van der Waals complex

We report here the fluorescence excitations spectrum of the Si–Ar van der Waals complex in the region of the (3p4s)3 P–(3p 2)3 P atomic transition. Long progressions are observed, which have been assigned to a Π–Σ transition. The potential curves derived from the analysis of these progressions are discussed in terms of effects of spin–orbit coupling on van der Waals interactions.

Energy partitioning in the VUV photodissociation of ClNO as a function of excitation energy

The fluorescence of NO Rydberg states resulting from the VUV photodissociation of ClNO with synchrotron radiation between 120 and 165 nm has been studied. The excitation spectra for the production of NO A 2Σ+ v′ = 0,1,2, C 2Π v′ = 0, D 2Σ+ v′ = 0 have been measured and show by comparison with the ClNO absorption spectrum a state‐to‐state correlation. The internal energy content (vibrational and rotational) of the NO A 2Σ+ fragment has been determined as a function of excitation energy. The rotational level distribution of NO(A) is not of Boltzmann type but follows a Gaussian‐like distribution as shown from the detailed analysis of the fluorescence spectrum obtained at 147 nm. The excess energy dependence of the rotational and vibrational energy disposal deviates strongly from statistical predictions and can be explained on the basis of a simple quasidiatomic impulsive model. The time decays of NO(A) have been recorded and the rate coefficients for quenching by ClNO was found to be larger for NO(A) resulti...

Photodissociation of methyl nitrite in the vacuum ultraviolet. II. Excitation energy dependence of the NO fluorescent fragments

The production efficiency of NO excited fragments in A 2Σ+ v′=0,1,2,3, C 2Π v′=0, D 2Σ+ v′=0 states resulting from the photolysis of methyl nitrite in the far UV has been measured as a function of incident wavelength using the synchrotron radiation of the electron storage ring of Orsay as the source of excitation. The experimental energy threshold to produce each NO excited state is in agreement with a bond dissociation energy of CH3O–NO of 1.8 eV. The excitation spectra of electronically excited NO A 2Σ+, C 2Π, D 2Σ+ are discussed in relation with the absorption spectrum of the parent molecule. The vibrational energy distribution in NO A 2Σ+ v′=0,1,2 can be explained by a statistical energy partition between CH3O and NO products provided some restrictions on the activated modes of the CH3O fragment. NO A 2Σ+ v′=3 population shows an anomalous behavior which can be indicative of a half‐collision effect due to the interaction of NO C 2Π and D 2Σ+ primary product with CH3O.