Cm Western

The B̃ 1E‘ state of ammonia: Sub‐Doppler spectroscopy at vacuum ultraviolet energies

Spectra of the 2n0 progression in the B 1E‘–X 1A’1 transition in NH3 and ND3 have been recorded at an effective resolution down to 0.008 cm−1 at the VUV equivalent energy. Line shape analysis reveals a predissociation lifetime of 0.25(2) ns for ND3 and 6.1(7) ps for NH3 independent of rotational state and v2. Measurement of the magnetic sensitivity of low J lines gave an electronic g value. Rotational analyses of the 2n0 bands with n up to 6 for ND3 and 8 in NH3 are presented and indicate some small vibrational perturbations. The observed difference between the Coriolis coupling constant ζ derived from the rotational structure and the g value indicates a mild Jahn–Teller effect.

Sub-doppler spectroscopy of the C′ v2 = 2 Rydberg level of ND3 at vacuum ultraviolet energies

A pulsed dye laser amplifier for cw lasers has been constructed. By frequency doubling the output pulses, and using counter-propagated beams with 2 + 1 multiphoton ionisation detection, very high resolution spectroscopic studies of atomic and molecular states lying at vacuum ultraviolet energies have become possible. The effective VUV two-photon bandwidth was determined from a sub-Doppler 6s7s 1S0−6s2 1S0 excitation spectrum of mercury vapour, and found to be 275 MHz (0.009 cm−1) fwhm. The 220 band in the C′1A′1X1A1 Rydberg transition of ND3 shows very narrow sub-Doppler linewidths, of which the narrowest, with J = K = 0, has a deconvoluted homogeneous width of 350 MHz in our experiment, corresponding to a lifetime of 0.45 ns. The general trend of an increase in linewidth with rotational excitation is consistent with rovibronic coupling to the continuum of the A1A″2 state. In addition, a number of apparently random level shifts and anomalously weakened lines is attributed to a perturbation, most probably by the overlapping and more severely predissociated v 2 = 8 level of the B 1E″ state, acting as an additional doorway to the dissociative A state levels.

Double-resonance multiphoton ionisation studies of Franck-Condon disfavoured vibronic levels of the B̃ and C̃ Rydberg states of ammonia

Multiphoton optical-optical double-resonance spectroscopy via predissociating vibronic levels of the A state has been used to reveal hitherto uncharacterised vibrational modes of the B and C′ Rydberg states of NH3 and ND3. The modes involved are ν1 in the C′ state and ν3 and ν4 in the B state; we present rotational analyses of ten bands involving these modes. We also describe a method for simulating the double-resonance spectra which takes account of the complications introduced by the short-lived nature of the intermediate state.

An optical-optical double resonance study of the [Btilde] 2Π i (0000) state of NCO

We report the use of optical-optical double resonance to obtain a rotational analysis of the previously unanalysable [Btilde] 2Π i (0000) state of NCO via the A 2∑+(0000) state. The complexity of the band is shown to arise out of perturbations by every A-state vibrational level with K leqslant; 2 in the vicinity of the [Btilde] 2Π i (0000) level. Two models are presented for this state. Firstly the 2Π1/2 and 2Π3/2 components are fitted independently with different values of B, D, p and q for the two sub-states of the [Btilde] 2Π(0000) state. These differences are removed in a second model which describes all the observed energy levels but at the expense of the addition of an arbitrary extra state not directly observed in our spectra that represents the mean interaction of other nearby A vibronic states.

Rydberg states of the SH(SD) radical revealed by multiphoton ionization spectroscopy

Three hitherto uncharacterized Rydberg states of the SD radical, each arising from a … 5σ22π24p 1 ← … 5σ22π3 electronic promotion, have been identified through analysis of the rotationally structured two photon resonance enhancements each provides to the multiphoton ionization spectrum of this species. One, possessing 2Σ- symmetry, had actually been observed (but not recognized as such) in an earlier absorption study. The others, of 2Π and 2Φ symmetry, have not been reported previously. The SH radical was also observed to exhibit fragmentary rotational structure in the wavelength regions of the 2Σ--X 2Π and 2Φ-X 2Π transitions. The results provide strong support for the ab initio theoretical analysis of the vertical electronic spectrum of the mercapto radical by Bruna and Hirsch (1987, Molec. Phys., 61, 1359).

Characterisation of the Ẽ′1A′1 Rydberg state of ammonia by resonance enhanced multiphoton ionisation spectroscopy

Abstract Progression of bands associated with the ν2 (bending) vibrational mode of the E ′ 1 A ′ 1 (4 pa ″ 2 ← 3 a 1 ) Rydberg states of both NH3 and ND3 have been observed as two-photon resonances in the multiphoton ionisation spectrum of these two isotopomers at excitation wavelengths in the range 248–275 nm. Band contour analysis yields excited state spectroscopic parameters and some insight into the predissociation behaviour of these levels, all of which show many parallels with previous knowledge concerning the C′1A′1 Rydberg state of ammonia (arising from the corresponding 3pa″2 ← 3a1 orbital promotion).

Hyperfine-induced breakdown of the ΔJ selection rule in NCO

Abstract The 14 N hyperfine coupling constants have been determined for the A 2 Σ + electronic state of NCO. The hyperfine splitting is much greater than the fine-structure splitting at low N , thereby accounting for the observation of transitions in double-resonance spectra which apparently violate the Δ J selection rule. The consequences of this for energy-transfer studies and dispersed fluorescence spectra are discussed.

Rotational analysis of the 2ν5 band of formaldehyde

The spectrum of the 2ν5 C–H stretching band of formaldehyde was measured by Barry et al. (Phys. Chem. Chem. Phys. 4, 445 (2002)) with FTIR spectroscopy. In the current work, this spectrum has been rotationally analysed with the help of PGOPHER, a general purpose program for simulating and fitting rotational spectra. The spectrum shows several perturbations and there is sufficient information available on the vibrational structure from a dispersed fluorescence spectra study by Bowens et al. (J. Chem. Phys. 104, 460 (1996)) and theoretical calculations by Burleigh et al. (J. Chem. Phys. 104, 480 (1996)) to allow the perturbing states to be identified and deperturb the energy levels. The resulting perturbation parameters and rotational constants enable the experimental line positions to be reproduced with an average error of 0.013 cm−1. Also presented is a list of line positions and relative intensities in the region around 1.76 µm.