Photofragmentation dynamics of N,N-dimethylformamide following excitation at 193 nm.

N,N-dimethylformamide, HCON(CH3)2, is a useful model compound for investigating the peptide bond photofragmentation dynamics. We report data from a comprehensive experimental and theoretical study into the photofragmentation dynamics of N,N-dimethylformamide in the gas phase at 193 nm. Through a combination of velocity-map imaging and hydrogen atom Rydberg tagging photofragment translational spectroscopy we have identified two primary fragmentation channels, namely, fission of the N-CO "peptide" bond and N-CH3 bond fission leading to the loss of CH3. The possible fragmentation channels leading to the observed products are rationalised with recourse to CASPT2 calculations of the ground and first few excited-state potential energy curves along the relevant dissociation coordinates, and the results are compared with the data from previous experimental and theoretical studies on the same system.

Gas-Phase Retro-Diels-Alder Reactions of Cyclohexene, 1-Methylcyclohexene, and 4-Methylcyclohexene following Photoexcitation at 193 nm: A Velocity-Map Imaging Study.

We present the results of a velocity-map imaging study into the retro-Diels-Alder (RDA) reactions of cyclohexene, 1-methylcyclohexene, and 4-methylcyclohexene following photoexcitation at 193 nm. Universal detection of all neutral fragments via vacuum-ultraviolet photoionization at 118 nm allows imaging of both RDA fragments in all cases. Fragment kinetic energy distributions reveal contributions from both dissociative ionization and retro-Diels-Alder reaction of the respective parent molecules, yielding reaction products with a high degree of internal excitation. Together with the observed isotropic product angular distributions, this is consistent with a mechanism in which the RDA reaction occurs from high vibrational levels of the electronic ground state following internal conversion from a higher-lying state initially populated in the photoexcitation process, as predicted by frontier molecular orbital theory. Velocity-map images and total translational energy distributions for the RDA products of 1-methylcyclohexene and 4-methylcyclohexene are very similar to those for unsubstituted cyclohexene, indicating that methyl substitution either adjacent to or far from the double bond has little effect on the dynamics of the RDA process.

Dynamics of the A-band ultraviolet photodissociation of methyl iodide and ethyl iodide via velocity-map imaging with 'universal' detection.

We report data from a comprehensive investigation into the photodissociation dynamics of methyl iodide and ethyl iodide at several wavelengths in the range 236-266 nm, within their respective A-bands. The use of non-resonant single-photon ionization at 118.2 nm allows detection and velocity-map imaging of all fragments, regardless of their vibrotational or electronic state. The resulting photofragment kinetic energy and angular distributions and the quantum yields of ground-state and spin-orbit excited iodine fragments are in good agreement with previous studies employing state-selective detection via REMPI. The data are readily rationalised in terms of three competing dissociation mechanisms. The dominant excitation at all wavelengths studied is via a parallel transition to the (3)Q0 state, which either dissociates directly to give an alkyl radical partnered by spin-orbit excited iodine, or undergoes radiationless transfer to the (1)Q1 potential surface, where it dissociates to an alkyl radical partnered by iodine in its electronic ground state. Ground state iodine atoms can also be formed by direct dissociation from the (1)Q1 or (3)Q1 excited states following perpendicular excitation at the shorter and longer wavelength region, respectively, in the current range of interest. The extent of internal excitation of the alkyl fragment varies with dissociation mechanism, and is considerably higher for ethyl fragments from ethyl iodide photolysis than for methyl fragments from methyl iodide photolysis. We discuss the relative advantages and disadvantages of single-photon vacuum-ultraviolet ionization relative to the more widely used REMPI detection schemes, and conclude, in agreement with others, that single-photon ionization is a viable detection method for photofragment imaging studies, particularly when studying large molecules possessing multiple fragmentation channels.

An introduction to velocity-map imaging mass spectrometry (VMImMS).

This account introduces a new variant of time-of-flight mass spectrometry, termed velocity-map imaging mass spectrometry (VMImMS). While the ion abundances recorded in conventional ToF-MS measurements are highly useful for molecular quantification and structure determination, the final parent and fragment ion yields are Largely blind to the dynamics of the processes in which the ions were formed inside the mass spectrometer. By recording the velocity distribution of each ion in tandem with the mass spectrum, not only can the details of the dissociative ionisation dynamics be unravelled, but the extra dimensions of information can be used for enhanced molecular fingerprinting, separating contributions from ions with identical mass-to-charge ratio and resolving components within mixtures, to name but a few examples. Measuring ion-velocity distributions within a mass spectrometry measurement is not new, but incorporating imaging techniques developed within the reaction dynamics community provides vastly improved velocity resolution for all ions simultaneously in a single-stage instrument. This account provides an introduction to VMImMS, outlines the fundamental instrumentation and detector requirements and the challenges associated with developing the method further, and details proof-of-concept work from our Laboratory on a number of potential applications of the technique.

Fragmentation dynamics of the ethyl bromide and ethyl iodide cations: a velocity-map imaging study.

The photodissociation dynamics of ethyl bromide and ethyl iodide cations (C2H5Br(+) and C2H5I(+)) have been studied. Ethyl halide cations were formed through vacuum ultraviolet (VUV) photoionization of the respective neutral parent molecules at 118.2 nm, and were photolysed at a number of ultraviolet (UV) photolysis wavelengths, including 355 nm and wavelengths in the range from 236 to 266 nm. Time-of-flight mass spectra and velocity-map images have been acquired for all fragment ions and for ground (Br) and spin-orbit excited (Br*) bromine atom products, allowing multiple fragmentation pathways to be investigated. The experimental studies are complemented by spin-orbit resolved ab initio calculations of cuts through the potential energy surfaces (along the RC-Br/I stretch coordinate) for the ground and first few excited states of the respective cations. Analysis of the velocity-map images indicates that photoexcited C2H5Br(+) cations undergo prompt C-Br bond fission to form predominantly C2H5(+) + Br* products with a near-limiting 'parallel' recoil velocity distribution. The observed C2H3(+) + H2 + Br product channel is thought to arise via unimolecular decay of highly internally excited C2H5(+) products formed following radiationless transfer from the initial excited state populated by photon absorption. Broadly similar behaviour is observed in the case of C2H5I(+), along with an additional energetically accessible C-I bond fission channel to form C2H5 + I(+) products. HX (X = Br, I) elimination from the highly internally excited C2H5X(+) cation is deemed the most probable route to forming the C2H4(+) fragment ions observed from both cations. Finally, both ethyl halide cations also show evidence of a minor C-C bond fission process to form CH2X(+) + CH3 products.

Multimass velocity-map imaging with the Pixel Imaging Mass Spectrometry (PImMS) sensor: an ultra-fast event-triggered camera for particle imaging.

We present the first multimass velocity-map imaging data acquired using a new ultrafast camera designed for time-resolved particle imaging. The PImMS (Pixel Imaging Mass Spectrometry) sensor allows particle events to be imaged with time resolution as high as 25 ns over data acquisition times of more than 100 µs. In photofragment imaging studies, this allows velocity-map images to be acquired for multiple fragment masses on each time-of-flight cycle. We describe the sensor architecture and present bench-testing data and multimass velocity-map images for photofragments formed in the UV photolysis of two test molecules: Br(2) and N,N-dimethylformamide.

A new detector for mass spectrometry: direct detection of low energy ions using a multi-pixel photon counter.

A new type of ion detector for mass spectrometry and general detection of low energy ions is presented. The detector consists of a scintillator optically coupled to a single-photon avalanche photodiode (SPAD) array. A prototype sensor has been constructed from a LYSO (Lu(1.8)Y(0.2)SiO(5)(Ce)) scintillator crystal coupled to a commercial SPAD array detector. As proof of concept, the detector is used to record the time-of-flight mass spectra of butanone and carbon disulphide, and the dependence of detection sensitivity on the ion kinetic energy is characterised.

RG+ formation following photolysis of NO-RG via the Ã-X transition: a velocity map imaging study.

Kr(+) and Xe(+) formation following photodissociation of NO-RG (RG = Kr or Xe) molecules via the Ã-X electronic transition in the 44,150-44,350 cm(-1) region has been investigated using velocity map imaging. Nuclear kinetic energy release (nKER) spectra indicate that the NO cofragment is produced in multiple vibrational states of the electronic ground state, with a high degree of rotational excitation. Photofragment angular distributions and nKERs are consistent with photo-induced charge transfer at the two-photon level followed by dissociative ionization at the three-photon level. RG(+) angular distributions showing highly parallel character relative to the laser polarization axis are indicative of a high degree of molecular alignment in the dissociating species.