W. Jeremy Jones

NO (C2Π-A2Σ+) emission during the radiative recombination of N and O atoms

Abstract The spectrum of the (0, 0) band in the C 2 Π-A 2 Σ + transition of NO has been observed for the first time at high resolution. The emission arises as a result of recombination of N and O atoms via inverse predissociation. The absence of lines from the lowest lying levels in C 2 Π(υ = 0) fixes the dissociation energy at 52400 ± 10 cm −1 , in excellent agreement with earlier values which were derived indirectly. The break-off in emission is clearly visible in spectra taken from 5 toor of N 2 indicating that N 2 electronically quenches NO C 2 Π(υ = 0) in preference to causing rotational relaxation.

Angular-dependent photoselection and anisotropy of optical absorption for photochromic molecules in glassy poly(methyl methacrylate)

It is shown that films of poly(methyl methacrylate) which contain the thermally-stable photochrome 7,7a-dihydro-2,4,7,7,7a-pentamethylbenzo-[b]-furan-5,6-dicarboxylic anhydride II may be rendered optically dichroic using polarized laser light (λ = 514·5nm). The dichroism is produced by angular-dependent photoselection of photochromic molecules in the photoconversion of photochrome II to fulgide I. The absorption anisotropy has been measured as a function of the angle between the planes of polarization of the pump and probe beams, and it is found to be time-invariant for this photochrome/glassy polymer system at room temperature. It is suggested that the time-dependence of absorption anisotropy of thermally-stable photochromic molecules in host polymer or liquid media should provide a direct means of studying their reorientational motions, and should be applicable over the time-range 10-11 to 10+3 s.

Concentration-modulated absorptiion spectroscopy.I

Abstract The absorption of photons from a high-intensity beam of laser radiation perturbs the concentration difference between the two states coupled by the incident radiation. This perturbation is investigated by a second beam using well-known pump and probe methods. By using picosecond mode-locked lasers and high-frequency modulation methods the sensitivity limit of the technique developed, termed concentration-modulated absorption spectroscopy, COMAS, is extended dramatically and compares most favourably with fluorescence excitation spectroscopy. The quantitative treatment of the theory shows that the gain created on the probe laser can be related formally to the transmittance of the sample, the relationship providing a direct measure of the concentration of the absorber. This COMAS approach to concentration measurement dispenses with the need for calibration using a known concentration of the chromophore or an accurate knowledge of the appropriate absorption coefficient. Accordingly, it is most suitable for investigating species concentrations in mixtures in the gaseous and in the liquid states. Experimental results using lithium in a hydrocarbon flame and thionin (Lauths violet) in aqueous solution are reported. Some of the wide ranging applications of this technique are discussed.

Motional narrowing in the v 1/2v 2 Fermi resonance diad of CO2

Studies of the v 1 and 2v 2 Q branches in the Raman spectrum of carbon dioxide have been carried out at a resolution of ∼30 MHz using the methods of Raman Amplification (Raman Gain) Spectroscopy. The v 1 band at 1286 cm-1 is not resolved at atmospheric pressure. The 2v 2 band, although similarly unresolved over the pressure range 120–760 Torr, is extremely narrow with a full width at half maximum (0·0076 cm-1 at 760 Torr) which is comparable with the Doppler width of a single transition and very markedly narrower than the width that would be expected of a single rotational S or O branch line at this pressure. This result is attributed to the effects of motional narrowing, which are thought to be very pronounced for 2v 2 because of the near coincidence of the individual J components within the Q branch.

Coherent amplification in fluorescent dye solutions. I. The fluorescence gain spectrum of cresyl violet

Abstract The gain spectrum of cresyl violet has been recorded over the wavelength range 590–650 nm using a synchronously-pumped mode-locked dye laser system. The measured gain spectrum is compared with that calculated using data derived from a conventional absorption spectrum. The value of the polarization techniques developed for measuring orientational and state lifetimes of dye molecules is demonstrated. By locating the probe laser in the spectral region associated with absorption or emission it is shown that it is possible to derive information on the ground or excited electronic states. Fluorescence lifetimes and orientational decay times for cresyl violet in methanol and ethanol are determined for the states S 0 and S 1 .

Rotational analysis of the 2A′→2A″ emission band system of HO2 at 1.43 µm

The HO22A′→2A″ emission band at 1.43 µm has been recorded at a resolution of 0.3 cm–1 by use of a SISAM interferometer. Only K′ < 2 levels are found. A full spectroscopic study has been carried out and rotational constants obtained for the 2A′ and 2A″ states, the latter being in agreement with those recently obtained using the laser paramagnatic resonance technique. The 1.51 µm emission and previously assigned to HO2 is found to be due almost entirely to the OH radical.

Vibration–rotation infrared emission spectrum of hydrogen isocyanide, HNC, at 2.75 µm

The infrared emission spectrum of HNC has been recorded using a SISAM interferometer. Excited-state HNC is formed by reaction of nitrogen atoms with methyl bromide in a fast-flow system at 1–3 Torr. The following transitions of HNC are observed near 3640 cm–1: 1000–0000, 1001–0001, 1110–0110 and 1200–0200.

Determination of the A0 rotational constant for C3v molecules. The ν4 Raman band of methyl iodide

The ν4 Raman band of methyl iodide has been photographed. The data obtained are sufficient to derive an unambiguous value of A0.

Direct observation of the 2.1Ag− electronic state of carotenoid molecules by consecutive two-photon absorption spectroscopy

Abstract The application of the methods of absorption—fluorescence gain to the study of canthaxanthin, β-carotene and lycopene, in solution in carbon tetrachloride, has led to the observation of the transition 2. 1 A g − −1. 1 A g − in absorption. The spectra arise as a result of consecutive two-photon absorption with the pump laser exciting 2. 1 A g − directly from the ground electronic state followed by absorption from the probe laser to a higher electronic state in the UV. The origin of 2. 1 A g − in canthaxanthin is identified as 15267±100 cm −1 . Although the probability of absorption in the first stage of this two-step process is extremely low, this consecutive two-photon absorption is observed because the next step has a very high transition probability. This fact, coupled with the sensitivity of the methods employed, renders this transition clearly visible in the region investigated.

Concentration-modulated absorption spectroscopy. Part 4.—The use of continuous-wave lasers

The concentration-modulation approach to the study of absorption spectra of atoms and molecules developed with mode-locked dye lasers is extended in this paper to continuous-wave lasers. A ‘gain’ theory is developed for use with c.w. lasers and a comparison is made between the ‘gain’ measurements of our c.w. experiments and those of earlier studies using pulsed lasers. The experimental agreement between the measured c.w. ‘gain’ and the pulsed ‘gain’, after correcting for differences between the experimental arrangements employed, provides strong support for the ‘gain’ theory developed here. It is clearly seen that by means of this theory and the derived equations it will be possible to evaluate species concentrations where the state lifetimes are known, or the lifetimes where the concentrations are known. These studies provide a firm foundation for expansion of the applicability of this highly sensitive absorption technique for kinetic and spectroscopic investigation of chemical systems.