Wavelength-dependent Coulomb explosion in carbon disulphide (CS2) clusters: generation of energetic multiply charged carbon and sulphur ions

Intensity dependence of multiply charged atomic ions from argon clusters in moderate nanosecond laser fields

We report the laser intensity dependence of multiply charged atomic ions (MCAIs) Arⁿ⁺ with 2 ≤ n ≤ 8 from argon clusters in focused nanosecond laser fields at 532 nm. The laser field, in the range of 10¹¹-10¹² W/cm², is insufficient for optical field ionization but is adequate for multiphoton ionization. The MCAI sections of the mass spectra for clusters containing 3700 and 26 000 atoms are dominated by Arⁿ⁺ with 7 ≤ n ≤ 9, extending to Ar¹⁴⁺. While the distributions of the MCAIs remain largely constant throughout the intensity range of the laser, the abundance of Ar⁺ relative to the abundances of the MCAIs increases dramatically with increasing laser intensity. Consequently, exponential fittings of the yields result in a larger exponent for Ar⁺ than for MCAIs, and the exponents of MCAIs with 2 ≤ n ≤ 8 are similar, with only slight variations for different charge states. The width of the arrival time and, hence, the corresponding kinetic energy of Ar⁺ also increases with increasing laser intensities, while the width of the arrival time of MCAIs remains constant throughout the range of measurements. These results call for more detailed theoretical investigations in this regime of laser-matter interactions.

Efficient coupling of nanosecond laser pulses with the cluster medium: Generation of hydrogen-like [C](5+) atomic ions

RATIONALE

Clusters exhibit diverse photochemical behavior as a function of laser parameters, i.e. wavelength, pulse duration and intensity. One such aspect of cluster photochemistry is the generation of energetic multiply charged atomic ions, upon efficient interaction of clusters with intense laser pulses. In the present work, mass spectrometric investigations have been carried out on clusters of tetrahydrofuran (THF, C4 H8 O) - a saturated cyclic ether - subjected to nanosecond laser pulse (spanning from UV to IR wavelength range) with the aim of shedding light on the complex mechanism of laser-cluster interactions, which is still ambiguous.

METHODS

THF clusters, generated via supersonic expansion of room-temperature THF vapours seeded in argon, were subjected to gigawatt intensity laser pulses (355, 532 and 1064 nm) obtained from a nanosecond Nd:YAG laser. The ions generated upon laser-cluster interaction were characterized using a time-of-flight mass spectrometer.

RESULTS

At 355 nm, THF clusters exhibit the usual multiphoton dissociation/ionization behavior while, at 532 nm, observation of multiply charged atomic ions of carbon (up to [C](4+) ) and oxygen (up to [O](3+) ) was ascribed to Coulomb explosion of THF clusters. For studies carried out at 1064 nm, multiply charged atomic ions of carbon up to [C](5+) having an ionization energy of ~392 eV were observed, at a laser intensity of 10(10) W/cm(2) .

CONCLUSIONS

The observation of [C](5+) atomic ions signifies efficient coupling of the laser energy with the cluster medium, using a nanosecond laser pulse. The results have been rationalized on the basis of a three-stage cluster ionization mechanism, suggesting the crucial role of the threshold laser intensity for initiating ionization within the cluster and generation of optimum charge centers for efficient extraction of energy from the laser pulse.

Effect of cluster expansion on photoionization of iron pentacarbonyl doped inert gas clusters under gigawatt intensity laser irradiation

RATIONALE

The aim of the investigation was to understand the variation in ionization dynamics of inert gas clusters upon doping with species with lower ionization energy than the inert gas constituent. It was postulated that the use of dopant species having lower ionization energy would lead to facile ionization of doped inert gas clusters, resulting in enhancement of the charge state of atomic ions compared with those obtained for pure inert gas clusters.

METHODS

Inert gas clusters (Ar(n), Kr(n) or Xe(n)) doped with iron pentacarbonyl were generated by supersonic expansion and subjected to gigawatt intensity laser pulses (266, 355, 532 and 1064 nm wavelengths) obtained from a nanosecond Nd:YAG laser. The ions generated upon laser-cluster interaction were characterized using a time-of-flight mass spectrometer.

RESULTS

Upon interaction of the laser with the doped inert gas clusters, the charge states of the atomic ions were found to increase with the laser wavelength. However, the highest observed charge states were found to be lower for doped inert gas clusters than for pure inert gas clusters, at all laser wavelengths.

CONCLUSIONS

Wavelength-dependent generation of multiply charged atomic ions has been explained based on the three stage model, i.e. multiphoton ionization ignited-inverse bremsstrahlung heating, and electron ionization. This model explains enhancement in the charge state of atomic ions with increasing wavelength based on inverse bremsstrahlung heating of the inner ionized electron, which is a more efficient process at longer wavelengths. Inefficient coupling of laser energy in the case of doped inert gas clusters compared with pure inert gas clusters has been rationalized on the basis of accelerated disintegration of the cluster due to facile initial ionization of dopant molecules having low ionization energy. The results suggest that a longer laser wavelength and a slower rate of cluster expansion facilitate the efficient transfer of optical energy into cluster systems.

Mass spectrometry in India

This review emphasizes the mass spectrometry research being performed at academic and established research institutions in India. It consists of three main parts covering the work done in organic, atomic and biological mass spectrometry. The review reveals that the use of mass spectrometry techniques started in the middle of the 20th century and was applied to research in the fields of organic, nuclear, geographical and atomic chemistry. Later, with the advent of soft and atmospheric ionization techniques it has been applied to pharmaceutical and biological research. In due course, several research centers with advanced mass spectrometry facilities have been established for specific areas of research such as gas-phase ion chemistry, ion-molecule reactions, proscribed chemicals, pesticide residues, pharmacokinetics, protein/peptide chemistry, nuclear chemistry, geochronological studies, archeology, petroleum industry, proteomics, lipidomics and metabolomics. Day-by-day the mass spectrometry centers/facilities in India have attracted young students for their doctoral research and other advanced research applications.

Multiphoton ionization and Coulomb explosion of C2H5Br clusters: a mass spectrometric and charge density study

Using time-of-flight mass spectrometry (TOFMS), laser-induced photochemistry of ethyl bromide clusters has been investigated at three different wavelengths (viz. 266, 355 and 532  nm) utilizing nanosecond laser pulses of ~5 × 10(9)  W/cm(2). An interesting finding of the present work is the observation of multiply charged atomic ions of carbon and bromine at 355 and 532  nm, arising from the Coulomb explosion of (C(2)H(5)Br)(n) clusters. At 266  nm, however, the (C(2)H(5)Br)(n) clusters were found to exhibit the usual multiphoton dissociation/ionization behaviour. The TOFMS studies are complemented by measuring the total charge density of the ionized volume at 266, 355 and 532  nm, using the parallel plate method, and the charge densities were found to be ~2 × 10(9), 6 × 10(9) and 2 × 10(11)  charges/cm(3), respectively. The significantly higher charge density and the presence of energetic, multiply charged atomic ions at 532  nm are explained by the higher ponderomotive energy of the 532  nm photon, coupled with the Coulomb stability of the residual multiply charged ethyl bromide clusters generated upon laser irradiation, due to their larger effective cluster size at 532  nm than at 355 and 266  nm.