G. Aravind

Absolute cross sections for dissociative electron attachment to H2O and D2O

The dissociative electron attachment (DEA) process to water (H2O) and heavy water (D2O) has been studied in the gas phase in a cross beam experiment for electron energies up to 20 eV. The apparatus used eliminates discrimination due to the kinetic energy and angular distribution of the ions. The cross sections are normalized to absolute values using the cross section for production of O− from O2 (Rapp and Briglia 1965 J. Chem. Phys. 43 1480). These are the first exhaustive measurements of absolute cross sections for both the H− and O− from H2O and D− and O− from D2O at all the three resonances. The results are compared with the scarce data available in the literature. Isotope effect is observed at the 12 eV resonance in the H− channel and at all the three resonances in the O− channel.

Spectral Tuning of the Photoactive Yellow Protein Chromophore by H-Bonding

Spectral tuning in the photoactive yellow protein (PYP) is investigated by performing gas-phase absorption measurements on a PYP-model chromophore with two water molecules hydrogen-bonded to it. The photoabsorption maximum shows an unusually large blue shift of 0.71 eV in going from the bare to the hydrogen-bonded chromophore. It is concluded that several interactions within the PYP protein are mutually cancelling each other, yielding an absorption maximum that is close to the absorption maximum of the bare chromophore. The system breaks apart upon photoexcitation in the gas phase by releasing the two water molecules, leaving the chromophore itself intact. The hydrogen-bonding interactions thus play an important role in stabilizing the gas phase chromophore against photofragmentation. The relaxation dynamics for the breakup process was also studied, and the timescale of relaxation via fragmentation was found to be < 25 ns.

Photodissociation pathways and lifetimes of protonated peptides and their dimers

Photodissociation lifetimes and fragment channels of gas-phase, protonated YA(n) (n = 1,2) peptides and their dimers were measured with 266 nm photons. The protonated monomers were found to have a fast dissociation channel with an exponential lifetime of ~200 ns while the protonated dimers show an additional slow dissociation component with a lifetime of ~2 μs. Laser power dependence measurements enabled us to ascribe the fast channel in the monomer and the slow channel in the dimer to a one-photon process, whereas the fast dimer channel is from a two-photon process. The slow (1 photon) dissociation channel in the dimer was found to result in cleavage of the H-bonds after energy transfer through these H-bonds. In general, the dissociation of these protonated peptides is non-prompt and the decay time was found to increase with the size of the peptides. Quantum RRKM calculations of the microcanonical rate constants also confirmed a statistical nature of the photodissociation processes in the dipeptide monomers and dimers. The classical RRKM expression gives a rate constant as an analytical function of the number of active vibrational modes in the system, estimated separately on the basis of the equipartition theorem. It demonstrates encouraging results in predicting fragmentation lifetimes of protonated peptides. Finally, we present the first experimental evidence for a photo-induced conversion of tyrosine-containing peptides into monocyclic aromatic hydrocarbon along with a formamide molecule both found in space.

Probing site selective fragmentation of molecules containing hydroxyl group using Velocity Slice Imaging

Site selective fragmentation is observed at the hydrogen site in small organic molecules like simple carboxylic acids and alcohols in the dissociative attachment (DA) process. We have investigated this site selectivity in the acetic acid and methanol molecules by measuring the kinetic energy and angular distribution of the hydride ions using Velocity Slice Imaging (VSI) technique. The results are explained in terms of the formation of valance excited Feshbach resonances. The kinetic energy and angular distribution measurements also reveal the dynamics associated with the process.

Absolute cross sections for dissociative electron attachment to water, methane and ammonia

Dissociative electron attachment process to water, methane and ammonia has been studied in gas phase at electron energies in the range of 0–20 eV. Absolute cross sections for production of negative ions in the DEA process to water, ammonia and methane have been measured in a cross beam experiment and compared with the data available in the literature.

Investigation of dissociative electron attachment to water using ion momentum imaging

The dynamics of the dissociative electron attachment (DEA) process to water (H2O) and heavy water (D2O) are investigated using the newly developed Velocity Map Imaging for low energy electron collisions. Where as the kinetic energy and angular distribution of the negative ion from water at the 6.5 eV resonance is found to be in excellent agreement with previous results, entirely different angular distributions are obtained at the two resonances at higher energies. We identify the 8.5 eV resonance as due to a 2B1 state in contrast to what has been believed till now based on experimental and theoretical studies. Large anisotropy in the angular distribution of H− and O− ions is observed at the third resonance indicating interesting dynamics involved.

Velocity Map Imaging of H- Ions from Dissociative Electron Attachment to H2O

The velocity map imaging (VMI) technique developed recently for dissociative electron attachment (DEA) experiments is improvised further to study the formation of H− ions. The experiment is used to study DEA to water molecules at the 6.5 eV and 8.5 eV resonances. The kinetic energy and angular distribution of the H− ions arising from this resonance are compared with those in the existing experimental and theoretical data.

Photodetachment studies with the linear time of flight photoelectron spectrometer

A linear time of flight photoelectron spectrometer was built for photodetachment studies on anions. The design of the newly constructed photoelectron spectrometer is discussed briefly. Angular distribution of photoelectrons in the photodetachment from OH−, As− and Cu− were measured at a few distinct wavelengths. Energy dependence of the asymmetry parameter was studied for these anions. The relation between electronic configuration and the energy dependence of the asymmetry parameter is briefly discussed.

Probing anion resonances in FeO−: a species of astrophysical relevance

Synopsis We report the first experimental probe on resonance states in FeO anion, a molecule of astrophysical relevance. Excitation of FeO− through collision with argon was performed to measure the kinetic energy release in the centre of mass frame. Results show access of two anion resonances upon collisional excitation. We discuss our evaluation of these two resonances from kinetic energy release in the centre of mass frame. The astrophysical importance of these anion resonances will be presented.

COLLISIONAL DESTRUCTION OF (n = 1 TO 4, 6) ANIONS OF ASTROPHYSICAL RELEVANCE

The stability of FeC− against dissociation in an astrophysical environment was probed by the collisional excitation of FeC−. Two anion resonances yielding Fe− and C− fragments were observed and studied through measurement of the kinetic energy released during fragmentation. The yield of Fe− was found to be nearly 5.5 times more than that of C− indicating the C− fragment to be in the loosely bound (2D) state. The possibility of avoided crossing leading to the observed fragment ion yield is also discussed. The dissociation of (n = 2 to 4, 6) cluster anions predominantly resulted in the cleavage of Fe–C bond yielding only fragments with similar energy release. The yield of is discussed in the light of the observed abundances of HC n in IRC+10216. The importance of rotational transitions pertaining to both the ground and excited-electronic states of these cluster anions is discussed.