Josef Pfab

Rotational distribution of CN from the 266 nm photodissociation of ICN

Abstract Rotational energy disposal in the 266 nm A state continuum photodissociation of ICN has been measured using pulsed laser photolysis wilh laser-induced fluorescence probing of the CN fragment. The rotational distribution is dominated by a cold part peaking at N = 1 or 2, and it has a high-temperature tail Possible dissociation mechanisms are discussed and a crude energy balance suggested.

Detection and probing of nitric oxide (NO) by two-colour laser photoionisation (REMPI) spectroscopy on the A ← X transition

Abstract A simple two-colour (1 + 1′) REMPI scheme for the detection and probing of nitric oxide (NO) via the A 2 Σ + ← X 2 Π transition has been introduced and characterised. Pulsed 226 and 308 nm laser light derived from a XeCl excimer pump laser has been used for resonant excitation and photoionisation. The two-colour (1 + 1′) ReMPI scheme has been found to offer substantially higher sensitivity than the conventional one-colour scheme. The reasons for this improvement and other advantages of the two-colour technique for the state-resolved probing of NO(X) are outlined.

Optogalvanic spectroscopy of argon and wavelength calibration in the near-ultraviolet

Abstract 29 optogalvanic lines of argon have been observed between 367 nm and 422 nm in a commercial hollow cathode tube. The analysis of the interference pattern arising from a quartz window requires only three reference points to allow direct wavelength calibration with high accuracy. The reported optogalvanic lines of argon yield an abundant number of reference points in the near-uv and extend the range of previously recorded optogalvanic lines of argon and neon in the visible. The version of the technique reported here provides an inexpensive and convenient yet highly accurate means for wavelength calibration in this region of the spectrum and is especially suitable for the calibration of dye or other tunable lasers.

Collisional deactivation of laser-excited acetylene by H2, HBr, N2 and CO

The deactivation of vibrationally excited acetylene by collisions with H2, HBr, N2 and CO was studied using the laser-induced fluorescence technique. After excitation of the coupled modes ν3 and (ν2+ν4+ν5) of C2H2 at 3300 cm–1 the time dependence of the fluorescence signals from ν2(1974 cm–1) and ν5(729 cm–1) was investigated. The rate constants for the deactivation of ν2 and ν5 by the collision partners CO and N2 were found to be relatively low and comparable with the known corresponding rate constants for rare-gas collisions. On the other hand the measured rate constants for the deactivation ν2 and ν5 by collisions with H2 and HBr are much higher. Additional measurements of CO fluorescence near 4.7 µm showed that the activation and deactivation of CO (v= 1) by collision with C2H2 is very effective. The influence of the measured rates on the C2H2 laser is considered briefly.

Geometry changes accompanying electronic excitation of nitrosomethane in the 650 nm region

The vapour phase absorption spectrum of nitrosomethane in the 600-720 nm region has been re-examined. The overall complexity of the spectrum is shown to be due to a conformational change from the eclipsed ground state to a staggered excited state. The prominent torsional features have been reassigned and are found to be consistent with a barrier of 500 ± 100 cm–1 to internal rotation in the excited state. Progressions of 390 and 1420 cm–1 have been assigned to the CNO bending and NO stretching modes in the excited state. Only one electronic absorption system with a Franck-Condon forbidden origin at 694.1 nm corresponding to the 1A″â†�1A′(n–, π*) transition has been observed.The geometry changes accompanying electronic excitation have been estimated using ab initio calculations for the 1A″ state. A substantial lengthening of the CN and NO bonds, a widening of their bond angle and a change in the phase of the torsional barrier are predicted.

Fourier-transform electronic absorption spectroscopy: The 800 nm near-infrared band system of nitrosyl cyanide (NCNO)

Abstract Dispersed and Fourier-transform spectra of NCNO vapour have been measured and some of the vibrational and coarse rotational structure has been analysed. The absorption is due to a 1 A″ ← 1 A′(n_,π*) transition with the origin at 882.2 nm. The main geometry change accompanying the transition consists of an opening up of the CNO angle.

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.

Electronic spectroscopy of the C state of NO by laser multiphoton ionization: rotational structure of the C2Pi(v'=0)«-X2Pi(v"=0) two-photon band

Rotationally resolved two-photon electronic spectra of nitric oxide (NO) on the C2Π(ν′= 0) ← X2Π(ν″ = 0) transition have been recorded in the near-UV between 384nm and 375nm by the (2+ 1) REMPI technique. The REMPI spectra of the δ(0,0) band have been measured and assigned for jet-cooled NO, NO at room temperature and for the nascent, highly rotationally excited NO formed by the UV-photodissociation of nitrosyl chloride (ClNO) in a one-colour dissociation-probe experiment. The wavelengths of all major rovibronic transitions have been measured accurately using optogalvanic spectroscopy and interference fringe counting as a calibration technique. The unperturbed transitions have been fitted to term values up to J = 53.5 with rotational energies in excess of 4000cm-1 to a single algebraic energy formula. The absence of certain Λ-doubled sub-branches in the spectra has been explained by different virtual two-photon routes via the electronic A state as the intermediate level. In connection with a discussion of...

Laser photoionisation (REMPI) spectroscopy of the 3s ← n Rydberg transition of jet-cooled acetaldehyde (CH3CHO)

Abstract The (2+1) resonance-enhanced multiphoton ionisation (REMPI) spectrum of jet-cooled acetaldehyde has been recorded over the wavelength range of 352 to 365 nm, and the vibrational assignments of the two-photon excitation spectrum have been extented to most of the prominent and many of the weaker vibronic features. The 3s ← n Rydberg nature of the transition and the preference for the eclipsed conformation of the excited state have been confirmed by the vibrational analysis of the observed spectra and by torsional level calculations.

Faraday communications. Electronic absorption spectrum of methyl nitrite in the near-ultraviolet

A re-examination of the structured near-UV electronic absorption spectrum of gaseous CH3ONO has revealed that previous assignments of the vibronic origin and of the prominent progression in the excited-state NO stretching vibration are insecure. An alternative assignment that is in better agreement with the observed spectra and their temperature dependence is presented.