Stuart M. Newman

Cavity ring-down spectroscopy

Cavity ring-down spectroscopy (CRDS) is a laser-based absorption spectroscopy technique that is starting to find extensive application as a consequence of the very high sensitivity of the method compared with more traditional absorption spectroscopy techniques. We describe the experimental implementation of CRDS and its application to a number of areas of research including laser diagnostics of hostile environments, reaction kinetics and spectroscopy, with particular emphasis on our ongoing studies of the fast (sub-nanosecond) predissociation of electronically excited states of small molecules and radicals.

Integrated absorption intensity and Einstein coefficients for the O2 a 1Δg–X 3Σg− (0,0) transition: A comparison of cavity ringdown and high resolution Fourier transform spectroscopy with a long-path absorption cell

The two experimental techniques of cavity ringdown spectroscopy and high-resolution, long-path Fourier transform spectroscopy have been used to measure quantitative absorption spectra and determine the integrated absorption intensity (Sint,B) for the O2 a 1Δg–X 3Σg− (0,0) band. Einstein A-factors and radiative lifetimes for the O2 a 1Δg v=0 state have been derived from the Sint,B values. The two methods give values for the integrated absorption intensity that agree to within 2%. The value recommended from the results of this study is Sint,B=3.10±0.10×10−24 cm molecule−1, corresponding to an Einstein-A coefficient of A=2.19±0.07×10−4 s−1 and a radiative lifetime of τrad=76 min. The measurements are in excellent agreement with the recent absorption study of Lafferty et al. [Appl. Opt. 37, 2264 (1998)] and greatly reduce the uncertainty in these parameters, for which accurate values are required for determination of upper stratospheric and mesospheric ozone concentrations.

Predissociation of the A 2Π3/2 state of IO studied by cavity ring-down spectroscopy

The IO A2Π3/2–X2Π3/2 (v′,0) bands with v′ = 0–5 and the (1,1), (2,1) and (3,1) hot bands have been investigated in absorption using cavity ring-down spectroscopy. Analysis of the spectra gives refined band origins and rotational constants for the v′ levels and reveals strongly v′-dependent predissociation rates for the A2Π3/2 state. Fitting of spectral lineshapes for the rotationally resolved (2,0) band shows that v′ = 2 undergoes a rotation-induced predissociation, most probably via coupling to a 2Σ− state, with lifetimes for the rotational levels that range from ca. 1 ns at J′ = 1.5 to 15 ps at J′ = 50.5. In contrast, however, the (0,0) and (3,0) bands, which are also rotationally structured, exhibit apparently J′-independent predissociation rates. The (1,0), (4,0) and (5,0) bands are sufficiently lifetime-broadened that no rotational structure is evident. Fits to the band contours give average homogeneous (FWHM) linewidths for the various vibrational bands of 0.30 ± 0.03 cm−1 for v′ = 0, 6 ± 1 cm−1 for v′ = 1, 0.80 ± 0.05 cm−1 for v′ = 3, 9 ± 2 cm−1 for v′ = 4 and 60 ± 10 cm−1 for v′ = 5. The dominant predissociation mechanism for v′ = 0,1,3,4, and 5 is attributed to spin–orbit coupling between the A2Π3/2 state and one or more Ω = 3/2 repulsive states.

Cavity ring-down spectroscopy of the A 2Π3/2–X 2Π3/2 transition of BrO

Abstract Cavity ring-down spectroscopy (CRDS) of the BrO A  2 Π 3/2 –X  2 Π 3/2 electronic transition is shown to be a sensitive laser-based detection method for this radical. Spectra of the A–X (7,0) and (12,0) bands show rotational structure and we report improved spectroscopic constants for the A  2 Π 3/2 v ′=7 and 12 levels and linewidths for the (7,0) and (12,0) transitions derived from fits to the band contours. The measured linewidths of 3.2±0.3 and 4.0±0.4 cm −1 correspond to predissociative lifetimes of the v ′=7 and 12 levels of 1.7±0.2 and 1.3±0.2 ps, respectively. From a knowledge of the reaction conditions and decrease in ring-down time on absorption we deduce an upper limit to the concentration of BrO radicals within the CRD chamber of 6×10 11 molecules cm −3 and, based on our signal-to-noise ratio, estimate a sensitivity in our experiment of 10 molecules cm −3 .

The UV absorption of ClO Part 1. The A2Π–X2Π spectrum at wavelengths from 285–320 nm studied by cavity ring-down spectroscopy

The UV absorption of ClO at wavelengths between 285 and 320 nm has been investigated using cavity ring-down spectroscopy. This wavelength region spans the (0,0) to (7,0) bands of the A2Π3/2–X2Π3/2 and A2Π1/2–X2Π1/2 transitions. The previously unobserved A2Π3/2–X2Π3/2 (0,0) and (1,0) absorption bands have been recorded with rotational resolution, and spectra of the (2,0) to (6,0) bands of the A2Π1/2–X2Π1/2 transition are shown for the first time. Analysis of the spectra gives refined band origins and rotational constants for the A2Π v′ levels and reveals a strong v′ dependence in the linewidths of rotational features. The lifetimes of the A2Π3/2v′=0–2 and A2Π1/2v′=2–4 levels are revised from previous estimates, and the lifetimes of A2Π1/2v′=5 and 6 levels have been determined. The deduced predissociation rates for A2Π3/2v′=3–7 confirm earlier studies. The lifetimes of vibrational levels of the two spin–orbit components of the A2ΠΩ state are markedly different. No evidence of J′-dependence in the predissociation is found, in contrast to the corresponding A2Π3/2 state of the IO radical.

Predissociation of the B 3Σu− state of S2

Predissociation of the B 3Σu− state of S2 has been investigated by a combination of cavity ring-down spectroscopy and model calculations. The experimental spectra of the B 3Σu−−X 3Σg−(v′,0) bands for 10⩽v′⩽22 span the wavenumber range 35 480–39 860 cm−1. Extensive variation is observed in the degree of rotational structure within the vibrational bands because of lifetime broadening caused by predissociation. Fits to the band contours give homogeneous linewidths for transitions to the B-state vibrational levels for 10⩽v′⩽17 that vary from ⩽1 cm−1 for the (10,0) band to 7±1 cm−1 for the (17,0) band with a maximum linewidth of 14±1 cm−1 for the (13,0) band. For v′⩾18, all bands are completely diffuse, indicating linewidths in excess of 15 cm−1. The experimental results are compared with the results of a theoretical model that uses a Rydberg–Klein–Rees (RKR) potential for the B 3Σu− state, ab initio calculations of the repulsive potentials that cross the B state, and Fermi golden rule calculations of the pred...