Multiply Charged Atomic Ions Research Articles

Kinetic energy distributions of atomic ions from disintegration of argon containing nanoclusters in moderately intense nanosecond laser fields: Coulomb explosion or hydrodynamic expansion.

We report kinetic energies (KE) of multiply charged atomic ions (MCAI) from interactions of moderately intense nanosecond lasers at 532 nm with argon containing clusters, including neat and doped clusters with a trace amount of trichlorobenzene. We develop a mathematical method to retrieve speed and thereby kinetic energy information from analyzing the time-of-flight profiles of the MCAI. This method should be generally applicable in detections of energetic charged particles with high velocities, a realm where velocity map imaging is inadequate. From this analysis, we discover that the KE of MCAI from doped clusters demonstrates a quadratic dependence on the charge of the atomic ions, while for neat clusters, the dependence is cubic. This result confirms the nature of the cluster disintegration process to be dominated by Coulomb explosion. This result bears more similarity to reports from extreme vacuum ultraviolet (EUV) fields with similar intensities, than to reports from near infrared (NIR) intense laser fields. However, the charge state distribution from our experiment is the opposite: we observe more higher charge state ions than reported in EUV fields, and our charge state distribution is actually similar to those reported in NIR fields. We also report a significant effect of the external electric field on the charge state distribution of the atomic ions: the presence of an electric field can significantly increase the charge from the atomic ions, as shown by a three-fold reduction in the average kinetic energy per charge. Although molecular dynamics simulations have been implemented for experiments in the EUV and NIR, our results allude to the need of a concerted effort in this regime of moderately intense nanosecond laser fields. The significant decrease in charge state distribution and the significant increase in KE from doped clusters, compared with neat clusters, is a telltale sign that the true interaction time between the laser field and the cluster may be substantially shorter than the duration of the laser, a welcome relief for molecular dynamics simulations.

Effects of aromatic molecules inside argon clusters on the formation of multiply charged atomic ions in moderately intense nanosecond laser fields.

We report the effect of two molecular species, fluorene (C13H10, Fl) and 1, 3, 5-trichlorobenzene (C6H3Cl3, 3ClB), embedded in atomic argon clusters, on the generation of multiply charged atomic ions (MCAIs) in moderately intense nanosecond laser fields at 532nm. The near resonant-enhancement of two photon absorption in the two aromatic species produces only a few low charge state (+2) atomic ions in a neat molecular cluster, but the enclosure of the same cluster with layers of Ar can significantly increase the charge state of MCAIs. The yields of singly charged atomic ions from the molecular species, such as H+, C+, and Cl+, are positively correlated with the number of molecules inside the Ar cluster, but the yields of the MCAI and Ar+ demonstrate opposite behaviors. A higher number of aromatic molecules are actually detrimental to the production of Ar+ and MCAIs. The results of exponential fittings of the yields of MCAIs at different laser intensities reveal a systematic change for the exponent of Ar+: with increasing concentrations of 3ClB in Ar clusters, the exponent decreases and eventually reaches the same value as those of MCAIs. These results are consistent with our previous hypothesis that the formation mechanism of MCAIs may be different from that of singly charged species and that the strong resonance of Ar3 + may play an important role in the overall energy absorption. Moreover, the effect of the molecular core seems to change the formation mechanism of Ar+ to that of MCAIs.

Wavelength dependent photoionisation of ethanol clusters: generation of hydrogen like C5+ ions at terawatt laser intensity.

Photoionisation of ethanol clusters has been investigated at different laser wavelengths over the intensity range of ∼1012-1013 W cm-2. Under 266 nm ionisation, singly charged fragments and protonated cluster ions (up to (C2H5OH)9H+) were observed in the mass spectrum. Moreover, small ion signals corresponding to doubly charged carbon ions (C2+) were also observed. At longer laser wavelengths (i.e. 355, 532 and 1064 nm), the charge states of multiply charged ions of C and O were found to increase systematically and the maximum observed charge states were C5+ and O6+ at 1064 nm. Generation of C5+ ions (IE: 392 eV) is noteworthy as it requires removal of a core shell electron from a carbon atom. A comprehensive study has been carried out to appreciate the ionisation mechanism of such higher charge state formation in an ethanol cluster at terawatt laser intensity. A laser power dependency study suggests the role of multiphoton ionsation in creating initial charge centres within the cluster. Subsequently the ionised electrons produced upon multiphoton ionisation gain energy from the laser field and cause further ionisation via electron impact ionisation. Energisation of electrons and generation of multiply charged atomic ions were found to depend on the stability of clusters in the laser field which in turn is related to intermolecular interactions of ethanol clusters. Moreover, anionic fragment ions were observed in the ionisation of ethanol clusters at ∼1012 W cm-2 which also influences the ionisation dynamics of the cluster.

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 532nm. 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 picosecond laser pulses with (CCl4)n clusters: Linear vs circular polarization

Ionic outcome and electron energy distribution, upon interaction of carbon tetrachloride clusters ((CCl4)n) with picosecond laser pulses of 1064 nm was investigated, to understand influence of linear and circular polarization on the mechanism of laser-cluster interaction. Contrary to the perception regarding higher proficiency of circular polarized laser pulses in collisional heating of ionized cluster, higher yield of multiply charged atomic ions (up to C5+ (I.E. ~ 392 eV) and Cl10+ (I.E. ~ 456.7 eV)) was obtained for linear polarization. Though marginally higher peak and mean electron energy were recorded for circular polarized laser pulses, with electron energy distribution extending up to ~ 300–350 eV. Present studies provide insight about the crucial steps of complex laser-cluster interaction mechanism, where inverse Bremsstrahlung energization of quasi-free electrons and screening effect within the ionized cluster are found to play a decisive role in determining efficiency of laser-cluster interaction.

Photochemistry of diethyl ether clusters at 1012 W/cm2: A mass spectrometry and charge density study

Photochemistry of diethyl ether clusters has been studied at discrete laser wavelengths spanning from UV to IR region at intensity ~1012 W/cm2 using time-of-flight mass spectrometry. At 266 nm, singly charged fragments, protonated cluster ions were observed due to multiphoton ionisation of clusters. Further, intra cluster ion–molecule reactions lead to generation of H2+, H3+ ions. In addition, C2+ ions were observed at this wavelength. With increase in laser wavelength to 355, 532 and 1064 nm, systematic increase in charge state of multiply charged atomic ions of oxygen, carbon was observed. Highest observed charge states were O6+, C4+ at 1064 nm. Mass spectrometric study was complemented by charge density measurement, which shows an increasing trend with wavelength supporting efficient ionisation of clusters at longer wavelength. Studies reveal that ionisation is initiated via multiphoton ionisation followed by electron ionisation. The multiple ionisation of diethyl ether clusters predominantly depends on the rate of energisation of electrons in the laser field and Coulomb stability of the cluster has insignificant contribution to the final ionic outcome.

Production of Multiply Charged Argon Ions in Moderate Nanosecond Laser Fields: An Open Question or a Forgone Conclusion?

We reply to the Viewpoint by Vatsa and Mathur on our publication reporting the observation of multiply charged atomic ions from argon clusters doped with aromatic chromophores in a moderate nanosecond laser field. Vatsa and Mathur raised three concerns about the proposed explanation and offered additional ideas for the reported process. We agree with some of their concerns and welcome the addition of information, and we also clarify a few misunderstandings of our intention, perhaps caused by our implicit assumption of contextual relations. While the experimental results are indisputable, the interpretation is still a topic of debate, subject to further experimental investigations and theoretical modeling.

Interaction of (CCl4)n clusters with nanosecond and picosecond laser pulses at discrete wavelengths spanning from IR to UV region: A comparative study using TOFMS

Using time-of-flight mass spectrometer (TOFMS), photochemistry of carbon tetrachloride clusters ((CCl4)n) has been investigated upon interaction with nanosecond and picosecond Nd:YAG laser pulses (266, 355, 532 and 1064 nm) spanning over intensity range of ∼1.2 × 109 to 7.9 × 1012 W/cm2. At all the laser wavelengths upon interaction of (CCl4)n clusters with laser pulse, multiphoton excitation leads to excessive fragmentation of cluster constituents, which are subsequently ionized by multiphoton absorption process. Depending on laser wavelength and pulse duration, at 266 and 355 nm these clusters exhibit dominant ion signal corresponding to singly charged fragment ions i.e. Cl+, CCl+, CCl2+ and CCl3+ in the mass spectra. In addition, for studies carried out at 266 nm using picosecond laser pulses minor ion signal corresponding to C2Cl+ and C2Cl2+ were also observed. Generation of these ions has been ascribed to intra-cluster radical chemistry resulting in generation of neutral precursors, which subsequently are multiphoton ionized. At 532 and 1064 nm, dominant ion signal corresponding to multiply charged atomic ions of carbon (up to C4+/C5+) and chlorine (up to C11+) have been observed. Though multiphoton absorption is the primary process driving the photochemistry of (CCl4)n clusters, observation of multiply charged atomic ions at 532 and 1064 nm has been ascribed to dominant role played by quasi-free electrons which are confined within the ionized cluster, in efficient coupling of laser energy at longer laser wavelengths via inverse Bremsstrahlung process.

Gas phase ionisation of carbon disulfide clusters at terawatt laser intensity: Generation of singly and multiply charged atomic and molecular ions

Gas phase ionisation of carbon disulphide clusters (CS2)n with different laser wavelengths of terawatt intensity has been studied using time-of-flight mass spectrometry. In the mass spectra, series of singly charged cluster fragments such as C+, S+, CS+, [C(CS)]+, [S(CS)]+, [S2(CS)]+, [C(CS2)]+, [C2(CS2)]+, [C3(CS2)]+, [S(CS2)]+, S2+, S3+, [CS(CS2)]+ etc. were observed at 266 nm photoionisation. Higher cluster fragments formation has been rationalised based on various ion-molecule reactions that occur within the cluster following ionisation. One of the prominent cluster fragment was S2+ whose formation mechanism has been explained based on two competitive ion-molecule reactions. Photoionisation at 355 nm leads to formation of doubly charged carbon and iodine ion along with singly charged cluster fragments. An increase in charge state of carbon and sulphur ions (up to +4 state) was observed on changing the laser wavelength to 532 nm, indicating efficient interaction of CS2 clusters at longer wavelength. In order to elucidate the mechanism of formation of multiply charged atomic ions at 355 and 532 nm, energy of electrons released on ionisation of CS2 cluster at different wavelengths was measured. Based on these experimental results, energisation of electrons within the cluster was observed due to influence of electric field of the laser pulse. The extent of energisation was found to be more at longer wavelength indicating role of electron impact ionisation for generation of multiply charged ions at this laser intensity conditions. Apart from multiply charged atomic ions, doubly charged molecular ion (CS22+) was also observed at 532 nm. Generation of CS22+ is explained based on similar ionisation mechanism as described for atomic multiply charged ions. However, lifetime of the doubly charged ion plays an important role for generation of the ions under our experimental conditions.

Looking inside the tunnelling barrier: I. Strong field ionisation from orbitals with high angular momentum in circularly polarised fields

Here and in the two companion papers we use time-dependent analytical R-Matrix (ARM) theory to consider the impact of electron-core interaction on (i) sensitivity of strong field ionisation to the sense of electron rotation in the initial state and (ii) electron spin polarisation. ARM is a rigorous approach to strong field ionisation that takes advantage of the semiclassical nature of electron dynamics in strong laser fields and naturally incorporates complex-valued classical trajectories in the time-propagation. While the appearance of complex time is associated with optical tunnelling, complex coordinates reflect non-adiabaticity of the electron response to the strong laser field. We focus on non-adiabatic electron-core interaction in the sub-barrier region and show how it affects ionisation from orbitals with high angular momentum ℓ = 2, 3 in multiply charged atomic ions with strong electron-core interaction. In this paper we discuss sub-cycle ionisation yields, while in the companion papers we focus on the effects of the long-range core potential on the ‘initial conditions’ (from the ARM perspective, companion paper II) for the photo electron as it emerges from the classically forbidden region and on the spin polarisation of the ejected electron (companion paper III).

Tailoring Ion Charge State Distribution in Tetramethyltin Clusters under Influence of Moderate Intensity Picosecond Laser Pulse: Role of Laser Wavelength and Rate of Energy Deposition.

Systematic manipulation of ionic-outcome in laser-cluster interaction process has been realized for studies carried out on tetramethyltin (TMT) clusters under picosecond laser conditions, determined by choice of laser wavelength and intensity. As a function of laser intensity, TMT clusters exhibit gradual enhancement in overall ionization of its cluster constituents, up to a saturation level of ionization, which was distinct for different wavelengths (266, 355, and 532 nm). Simultaneously, systematic appearance of higher multiply charged atomic ions and shift in relative abundance of multiply charged atomic ions towards higher charge state was observed, using time-of-flight mass spectrometer. At saturation level, multiply charged atomic ions up to (C2+, Sn2+) at 266 nm, (C4+, Sn4+) at 355 nm, and (C4+, Sn6+) at 532 nm were detected. In addition, at 355 nm intra-cluster ion chemistry within the ionized cluster leads to generation of molecular hydrogen ion (H2+) and triatomic molecular hydrogen ion (H3+). Generation of multiply charged atomic ions is ascribed to efficient coupling of laser pulse with the cluster media, facilitated by inner-ionized electrons produced within the cluster, at the leading edge of laser pulse. Role of inner-ionized electrons is authenticated by measuring kinetic energy distribution of electrons liberated upon disintegration of excessively ionized cluster, under the influence of picosecond laser pulse. Graphical Abstract ᅟ.

EUV emission spectra in collisions of highly charged tantalum ions with nitrogen and oxygen molecules

We have performed ion beam collision experiments using multiply charged tantalum ions and observed EUV (extreme ultra-violet) emission spectra in collisions of ions with molecular targets, N2 and O2. Broad UTAs (un-resolved transition arrays) from multiply charged Ta ions were observed, and the mean wavelengths of the UTAs shifted and became shorter at higher charge statea of Ta ions. These UTAs may be attributed to the 4f–5d and 4f–5g transitions. Not only the UTA emission from incident ions, but also the sharp emission lines from multiply charged fragment atomic ions were observed. Production of temporary highly charged molecular ions, their kinetic energy and fragmentation processes have been investigated with coincident detection technique. However, the observation of emission from the fragments might be for the first time. The formation mechanisms of the multiply charged fragment atomic ions from target molecules are discussed.

Mass spectrometric and charge density studies of organometallic clusters photoionized by gigawatt laser pulses.

Clusters on exposure to nanosecond laser pulses of gigawatt intensity exhibit a variety of photo-chemical processes such as fragmentation, intracluster reaction, ionization, Coulomb explosion, etc. Present article summarizes the experimental results obtained in our laboratory utilizing time-of-flight mass spectrometer which deal with one such aspect of cluster photochemistry related to generation of multiply charged atomic ions upon excessive ionization of cluster constituents (Coulomb explosion) at low intensity laser field (∼109 W/cm2 ). To understand the mechanism of laser-cluster interaction, laser as well as cluster parameters were varied. Mass spectrometric studies were carried out at different laser wavelength as well as varying the nature of cluster constituents, backup pressure, nozzle diameter, etc. In addition, charge density measurements were also preformed to get information about the total number of ions generated upon laser-cluster interaction as a function of laser wavelength. In case of pure molecular clusters, the charge state of atomic ions as well as charge density was observed to enhance with increasing laser wavelength, signifying efficient coupling of the cluster medium with nanosecond laser pulse at longer wavelength. While in case of clusters doped with species having comparatively lower ionization energy, the efficiency of laser-cluster interaction was less, in contrast to studies carried out using femtosecond lasers. Results obtained in the present work have been rationalized on the basis of proposed three-stage cluster ionization mechanism, that is, multiphoton ionization ignited-inverse Bremsstrahlung heating and electron ionization. © 2015 Wiley Periodicals, Inc. Mass Spec Rev. 36:188-212, 2017.

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

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. 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. 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) . 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.

Diverse photochemical behavior of dibromodifluoromethane (CF2Br2) monomer and cluster under gigawatt intensity laser fields

Dibromodifluoromethane (CF2Br2) monomers and clusters were irradiated with nanosecond laser pulses of intensity ∼109 W cm−2 at 266 nm, 355 nm and 532 nm and the ionic outcome was studied using time-of-flight mass spectrometry. Based on the monomer study at different wavelengths, it was inferred that CF2Br2 undergoes dissociation followed by ionization and the primary dissociation leads to the formation of CF2Br radical and atomic bromine. However, the cluster study reveals an altogether different photoionization channel. At 266 nm, CF2Br+ and Br2+ were observed in photoionization of CF2Br2 clusters and an intra-cluster photochemical reaction is responsible for the generation of these ions. Moreover, multiply charged ions of carbon, fluorine and bromine were observed at 355 nm and 532 nm. The mechanism of multiply charged ion formation due to the interaction of CF2Br2 clusters under a gigawatt intensity laser field has been discussed. Further, negative ions were detected in the photoionization of the CF2Br2 cluster at 532 nm and the effect of negative ions on the generation of multiply charged atomic ions will also be discussed.

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

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. 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. 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. 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.

Photochemistry of (CH3SCH3) n and (CH3SSCH3) n clusters at 355 and 532 nm using time-of-flight mass spectrometer

Photofragmentation/ionization of dimethyl sulphide (DMS) and dimethyl disulphide (DMDS) clusters, i.e., (CH3SCH3)n and CH3SSCH3)n, subjected to nanosecond laser pulses of 355 and 532 nm of intensity ~5 × 109 W/cm2, has been studied using time-of-flight mass spectrometry. At 355 nm, these clusters were found to exhibit multiphoton dissociation/ionization behavior similar to that reported earlier for their corresponding monomer. In contrast at 532 nm, these clusters were found to exhibit Coulomb explosion phenomenon resulting in generation of multiply charged atomic ions of carbon (up to C4+) and sulphur (S6+), along with other fragment ions arising from multiphoton dissociation/ionization of DMS and DMDS clusters.

Dimensional scaling treatment with relativistic corrections for stable multiply charged atomic ions in high-frequency super-intense laser fields

We present a theoretical framework which describes multiply charged atomic ions, their stability within super-intense laser fields, and also lay corrections to the systems due to relativistic effects. Dimensional scaling calculations with relativistic corrections for systems: H, H(-), H(2 -), He, He(-), He(2 -), He(3 -) within super-intense laser fields were completed. Also completed were three-dimensional self consistent field calculations to verify the dimensionally scaled quantities. With the aforementioned methods the system's ability to stably bind "additional" electrons through the development of multiple isolated regions of high potential energy leading to nodes of high electron density is shown. These nodes are spaced far enough from each other to minimize the electronic repulsion of the electrons, while still providing adequate enough attraction so as to bind the excess electrons into orbitals. We have found that even with relativistic considerations these species are stably bound within the field. It was also found that performing the dimensional scaling calculations for systems within the confines of laser fields to be a much simpler and more cost-effective method than the supporting D = 3 SCF method. The dimensional scaling method is general and can be extended to include relativistic corrections to describe the stability of simple molecular systems in super-intense laser fields.

Interaction of nanosecond laser pulse with tetramethyl silane (Si(CH3)4) clusters: Generation of multiply charged silicon and carbon ions

Present work reports significantly high levels of ionization, eventually leading to Coulomb explosion of Tetramethyl silane (TMS) clusters, on interaction with laser pulses of intensity ∼109 W/cm2. Tetramethyl silane clusters, prepared by supersonic expansion were photoionized at 266, 355 or 532 nm and the resultant ions were detected using time-of-flight mass spectrometer. It is observed that wavelength of irradiation and the size of the cluster are crucial parameters which drastically affect the nature of charge species generated upon photoionization of cluster. The results show that clusters absorb significantly higher energy from the laser field at longer wavelengths (532 nm) and generate multiply charged silicon and carbon ions which have large kinetic energies. Further, laser-cluster interaction at different wavelengths has been quantified and charge densities at 266, 355 and 532 nm are found to be 4x 1010, 5x 1010 and 5x 1011 charges/cm3 respectively. These unusual results have been rationalized based on dominance of secondary ionization processes at 532 nm ultimately leading to Coulomb explosion of clusters. In another set of experiments, multiply charged ions of Ar (up to +5 state) and Kr (up to +6 state) were observed when TMS doped inert gas clusters were photoionized at 532 and 355 nm. The extent of energy absorption at these two wavelengths is clearly manifested from the charge state of the atomic ions generated upon Coulomb disintegration of the doped cluster. These experiments thus demonstrate a novel method for generation of multiply charged atomic ions of inert gases at laser intensity of ∼ 109 W/cm2. The average size of the cluster exhibiting Coulomb explosion phenomena under giga watt intensity conditions has been estimated to be ∼ 6 nm. Experimental results obtained in the present work agree qualitatively with the model proposed earlier [D. Niu, H. Li, F. Liang, L. Wen, X. Luo, B. Wang, and H. Qu, J. Chem. Phys. 122, 151103(2005)] and point towards interaction of quasi-free electrons, generated during primary multiphoton ionization step, with a given wavelength in the presence of Coulombic field.

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.