Ion imaging methods are making ever greater impact on studies of gas phase molecular reaction dynamics. This article traces the evolution of the technique, highlights some of the more important breakthroughs with regards to improving image resolution and in image processing and analysis methods, and then proceeds to illustrate some of the many applications to which the technique is now being applied--most notably in studies of molecular photodissociation and of bimolecular reaction dynamics.

Criegee intermediates are important species formed during the ozonolysis of alkenes. Reaction of stabilized Criegee intermediates with various species like SO2 and NO2 may contribute significantly to tropospheric chemistry. In the laboratory, self-reaction can be an important loss pathway for Criegee intermediates and thus needs to be characterized to obtain accurate bimolecular reaction rate coefficients. Cavity ring-down spectroscopy was used to perform kinetic measurements for various reactions of CH2OO at 293 K and under low pressure (7 to 30 Torr) conditions. For the reaction CH2OO + CH2OO (8), a rate coefficient k8 = (7.35 ± 0.63) × 10(-11) cm(3) molecule(-1) s(-1) was derived from the measured CH2OO decay rates, using an absorption cross section value reported previously. A rate coefficient of k4 = (3.80 ± 0.04) × 10(-11) cm(3) molecule(-1) s(-1) was obtained for the CH2OO + SO2 (4) reaction. An upper limit for the unimolecular CH2OO loss rate coefficient of 11.6 ± 8.0 s(-1) was deduced from studies of reaction (4). SO2 catalysed CH2OO isomerization or intersystem crossing is proposed to occur with a rate coefficient of (3.53 ± 0.32) × 10(-11) cm(3) molecule(-1) s(-1).

Continuous wave (cw) diode lasers are increasingly being used as light sources in the visible and near-IR regions of the spectrum for cavity ringdown spectroscopy (CRDS) and cavity enhanced absorption spectroscopy (CEAS); the latter technique is also widely known as integrated cavity output spectroscopy (ICOS). The very high sensitivities to weak absorptions that are possible with cw CRDS and CEAS, coupled with the quantitative nature of the absorption measurements, are enabling a rapidly expanding range of applications. We review the benefits and practical implementation of these techniques; methods of data analysis for extraction of quantitative absorption data; the sensitivities of cw CRDS and CEAS, and how they might be optimised; and applications of cw CRDS and CEAS in molecular spectroscopy, atmospheric chemistry, plasma and flame chemistry, analytical science, and medical diagnosis via breath analysis. The development of CRDS and CEAS techniques exploiting cw diode lasers and, very recently, high luminosity light-emitting diodes, has stimulated a wealth of highsensitivity measurements. Highlights include quantitative measurement of various ultra-trace gases such as: NO 3 , NO 2 and ethene in ambient air samples; CO 2 isotopologues, ethane and other organic compounds in human breath samples; and excited electronic states of N 2 and O 2 in plasmas and discharges. Exciting developments include wavelength extension into the mid-IR and UV regions, and use of novel locked-cavity techniques to increase data acquisition rates and sensitivities.

Abstract:The Criegee intermediate acetone oxide, (CH 3 ) 2 COO, is formed by laser photolysis of 2,2-diiodopropane in the presence of O 2 and characterized by synchrotron photoionization mass spectrometry and by cavity ringdown ultraviolet absorption spectroscopy. The rate coefficient of the reaction of the Criegee intermediate with SO 2 was measured using photoionization mass spectrometry and pseudo-first order methods to be (7.3 ± 0.5) × 10 -11 cm 3 s -1 at 298 K and 4 Torr and (1.5 ± 0.5) × 10 -10 cm 3 s -1 at 298 K and 10 Torr (He buffer). These values are similar to directly measured rate coefficients of anti-CH 3 CHOO with SO 2 , and in good agreement with recent UV absorption measurements. The measurement of this reaction at 293 K and slightly higher pressures (between 10 Torr and 100 Torr) in N 2 from cavity ringdown decay of the ultraviolet absorption of (CH 3 ) 2 COO yielded even larger rate coefficients, in the range (1.84  0.12) × 10 -10 to (2.29 ± 0.08) × 10 -10 cm 3 s -1 . Photoionization mass spectrometry measurements with deuterated acetone oxide at 4 Torr show an inverse deuterium kinetic isotope effect, k H /k D = (0.53 ± 0.06), for reactions with SO 2 , which may be consistent with recent suggestions that the formation of an association complex affects the rate coefficient. The reaction of (CD 3 ) 2 COO with NO 2 has a rate coefficient at 298 K and 4 Torr of (2.1 ± 0.5) × 10 -12 cm 3 s -1 (measured with photoionization mass spectrometry), again similar to the reaction of anti-CH 3 CHOO with NO 2 . Cavity ringdown measurements of the acetone oxide removal without added reagents display a combination of first-and second-order decay kinetics, which can be deconvolved to derive values for both the self-reaction of (CH 3 ) 2 COO and its unimolecular thermal decay. The inferred unimolecular decay rate coefficient at 293 K, (305 ± 70) s -1 , is similar to determinations from ozonolysis. The present measurements confirm the large rate coefficient for reaction of (CH 3 ) 2 COO with SO 2 and the small rate coefficient for its reaction with water. Product measurements of the reactions of (CH 3 ) 2 COO with NO 2 and with SO 2 suggest that these reactions may facilitate isomerization to 2-hydroperoxypropene, possibly by subsequent reactions of association products.

(2015) Probing the ultrafast energy dissipation mechanism of the sunscreen oxybenzone after UVA irradiation. Journal of Physical Chemistry Letters, 6 (8). pp. 1363-1368. Permanent WRAP URL:http://wrap.warwick.ac.uk/83198 Copyright and reuse:The Warwick Research Archive Portal (WRAP) makes this work by researchers of the University of Warwick available open access under the following conditions. Copyright © and all moral rights to the version of the paper presented here belong to the individual author(s) and/or other copyright owners. To the extent reasonable and practicable the material made available in WRAP has been checked for eligibility before being made available.Copies of full items can be used for personal research or study, educational, or not-for profit purposes without prior permission or charge. Provided that the authors, title and full bibliographic details are credited, a hyperlink and/or URL is given for the original metadata page and the content is not changed in any way. Publisher's statement:This document is the Accepted Manuscript version of a Published Work that appeared in final form in Journal of Physical Chemistry Letters, copyright © American Chemical Society after peer review and technical editing by the publisher.To access the final edited and published work see http://dx.doi.org/10.1021/acs.jpclett.5b00417 A note on versions:The version presented here may differ from the published version or, version of record, if you wish to cite this item you are advised to consult the publisher's version. Please see the 'permanent WRAP url' above for details on accessing the published version and note that access may require a subscription. ABSTRACT: Oxybenzone is a common constituent of many commercially available sunscreens providing photoprotection from ultraviolet light incident on the skin. Femtosecond transient electronic and vibrational absorption spectroscopies have been used to investigate the non-radiative relaxation pathways of oxybenzone in cyclohexane and methanol after excitation in the UVA region. The present data suggest that the photoprotective properties of oxybenzone can be understood in terms of an initial ultrafast excited state enol  keto tautomerization, followed by efficient internal conversion and subsequent vibrational relaxation to the ground state (enol) tautomer.TOC GRAPHIC