Fragmentation dynamics of diatomic molecules under proton impact: Kinetic energy release spectra of CO^{q+} and NO^{q+} (q = 2, 3) molecular ions

We report on the fragmentation dynamics of triply charged, diatomic, molecular ions of NO and CO. Dissociative fragmentation after multiple ionization of NO and CO is studied under the impact of 200 keV proton beam using recoil-ion momentum spectrometer. Kinetic Energy Release distributions (KERDs) for various fragmentation channels were obtained. We have also calculated the potential energy curves (PECs) for ground and several excited states of NO^{3+} and CO^{3+} molecular ion. The obtained KERDs are discussed in the background of the calculated PECs as well as the simple Coulomb excitation model. Coulomb breakup of the unstable precursor molecular ion shows a clear preference for the N^{2+} + O^+ (and C^{2+} + O^+) fragmentation channel.Comment: 8 pages, 6 figures, 3 table

Strong fields induce ultrafast rearrangement of H-atoms in H2_2O

H-atoms in H$_2$O are rearranged by strong optical fields generated by intense, 10 fs laser pulses to form H$_2^+$, against prevailing wisdom that strong fields inevitably lead to multiple molecular ionization and the subsequent Coulomb explosion into fragments. This atomic rearrangement is shown to occur within a single 10 fs pulse. Comparison with results obtained with $\sim$300-attosecond long strong fields generated using fast Si$^{8+}$ ions helps establish thresholds for field strength and time required for such rearrangements. Quantum-chemical calculations reveal that H$_2^+$ originates in the $^1$A state of H$_2$O$^{2+}$ when the O-H bond elongates to 1.15 a.u. and the H-O-H angle becomes 120$^o$. Bond formation on the ultrafast timescale of molecular vibrations (10 fs for H$_2^+$) has hitherto not been reported.Comment: Submitted to Physical Review Lotter

Single and multiple ionization of CS<SUB>2</SUB> in intense laser fields: wavelength dependence and energetics

Single and multiple ionization of carbon disulphide by intense picosecond laser fields is the subject of this paper. Mass spectra were measured at five wavelengths from the infrared to the ultraviolet. In terms of the Keldysh adiabaticity parameter, we cover both the multiphoton and the tunnelling regimes. The dynamics of the dissociative ionization process is shown to be dependent upon the regime in which the laser - molecule interaction occurs. Resonances, which may be possible and which could access electronically excited states of the molecule, appear to play little part in the dynamics. Ion abundances have been measured as a function of laser intensity in the tunnelling regime; no correlation is found between measured values of saturation intensity and zero-field molecular properties such as dissociation or ionization thresholds and ionization energies. In addition, the covariance mapping technique is applied to study the dissociation dynamics of multiply charged ions at 1064 nm. The measured values of kinetic energy release accompanying formation of fragment ion-pairs are very much less than those measured in single-photon and electron-impact experiments. It is postulated that this reduction may be a manifestation of the extent to which potential energy surfaces of CS24+ ions are `flattened' by the action of the intense, linearly polarized laser radiation, akin to the bond-softening process that has been observed in the case of diatomic molecules. Our observations indicate that distortion of molecular potential energy surfaces may be the dominating feature in intense laser - molecule interactions

Measurements of dication lifetimes by translational energy spectrometry

A new application of mass spectrometry to experimental studies of molecular dynamics has been developed and implemented in which ion translational energy spectrometry at very high angular resolutions is utilized to study the unimolecular dissociation of metastable dications. A brief overview is presented of the experimental technique, which utilizes a 'home-made', three-sector (equivalent to BEE geometry) instrument. The fragmention kinetic energy spectrum obtained using such apparatus is capable of yielding information on the lifetimes of specific quantal states of dications of a wide variety of stable and radical species; two illustrative examples (CO2+ and SH2+) are presented

On the determination of the lifetime of metastable doubly charged molecules by ion translational energy spectrometry: CO<SUP>2+</SUP>

Ion translational energy spectrometry has been applied to study the kinetic energies of fragment ions resulting from spontaneous (unimolecular) dissociation of doubly charged molecules. Analysis of the shapes of the kinetic energy distributions yields information on the lifetime of the precursor molecular ion. A value of 1.0+or-0.25 mu s is deduced for the lifetime of CO<SUP>2+</SUP> ions

Lifetime measurements by ion translational energy spectrometry: metastable SH<SUP>2+</SUP> ions

Ion translational energy spectrometry has been used to determine the lifetime against spontaneous (unimolecular) dissociation of doubly charged molecular ions by studying the kinetic energies of fragment ions. Analysis of the shapes of the fragment ion kinetic energy distributions yields information on the lifetime of the precursor molecular ion. A value of 0.75+or-0.25 mu s is deduced for the lifetime of SH<SUP>2+</SUP> ions. Correlation of this value with the shapes of calculated potential energy curves of low-lying electronic states of SH<SUP>2+</SUP> enables deductions to be made of the importance of spin-orbit induced coupling between the lowest <SUP>4</SUP> Sigma <SUP>-</SUP> and <SUP>2</SUP> Pi states and provides information on the vibrational level population in the doubly charged molecular ion

Dissociation of highly charged N<SUB>2</SUB><SUP>q+</SUP> (q&gt;=2) ions via non-Coulombic potential energy curves

The kinetic energies released (KER) upon dissociation of N2q+ (q=2-10) ions are investigated by means of large scale, all-electron, ab initio, configuration interaction molecular orbital calculations. Results indicate that dissociation via non-Coulombic potential energy curves of N2q+ ions yields lower values of KER than would be expected from purely Coulombic considerations due to the significance of electronic charge density distributions in the internuclear region

A long-lived triply charged diatomic ion, CS<SUP>3+</SUP>

A metastable, triply charged, diatomic molecular ion, CS<SUP>3+</SUP>, has been discovered in collisions of O<SUP>-</SUP> and Si<SUP>-</SUP> with CS<SUB>2</SUB> at an impact energy of 40-70 keV. Concomitant configuration interaction calculations of potential energy curves and electron density distributions have been carried out which indicate that the lowest-energy <SUP>2</SUP> Sigma<SUB> g</SUB> state of this molecular ion is metastable for at least 3 mu s. Predictions are made of the values of the kinetic energies released upon dissociation of the lowest-energy <SUP>2,4,6</SUP> Sigma <SUB>g</SUB> and <SUP>2,4</SUP> Pi <SUB>g </SUB>states of CS<SUP>3+</SUP>

Hard X-ray generation from microdroplets in intense laser fields

Microdroplets of 15-&#956;m diameter are subjected to ultra-short laser pulses of intensities up to 1015Wcm−2 to produce hot dense plasma. The hot electrons produced in the microdroplet plasma result in efficient generation of hard X-rays in the range 50–150 keV at an irradiance as low as 8×1014Wcm−2. The X-ray source efficiency is estimated to be about 2 ×10−7%. A prepulse that is about 11ns ahead of the main pulse strongly influences the droplet plasma and the resulting X-ray emission. For a similar laser prepulse and intensity, no measurable hard X-ray emission is observed when the laser is focused on a solid target of similar composition and this indicates that liquid droplet targets are best suited for hard X-ray generation in laser–plasma interactions