Ionization and Coulomb explosion of Xenon clusters by intense, few-cycle laser pulses

Intense, ultrashort pulses of 800 nm laser light (12 fs, $\sim$4 optical cycles) of peak intensity 5$\times$10$^{14}$ W cm$^{-2}$ have been used to irradiate gas-phase Xe$_n$ clusters ($n$=500-25,000) so as to induce multiple ionization and subsequent Coulomb explosion. Energy distributions of exploding ions are measured in the few-cycle domain that does not allow sufficient time for the cluster to undergo Coulomb-driven expansion. This results in overall dynamics that appear to be significantly different to those in the many-cycle regime. One manifestation is that the maximum ion energies are measured to be much lower than those obtained when longer pulses of the same intensity are used. Ion yields are cluster-size independent but polarization dependent in that they are significantly larger when the polarization is perpendicular to the detection axis than along it. This unexpected behavior is qualitatively rationalized in terms of a spatially anisotropic shielding effect induced by the electronic charge cloud within the cluster

Strong light fields coax intramolecular reactions on femtosecond time scales

Energetic H$_2^+$ ions are formed as a result of intra-molecular rearrangement during fragmentation of linear alcohols (methanol, ethanol, propanol, hexanol, and dodecanol) induced by intense optical fields produced by 100 fs long, infrared, laser pulses of peak intensity 8$\times10^{15}$ W cm$^{-2}$. Polarization dependent measurements show, counterintuitively, that rearrangement is induced by the strong optical field within a single laser pulse, and that it occurs before Coulomb explosion of the field-ionized multiply charged alcohols

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

Probing interatomic potentials by ion translational energy spectrometry: a new crossed molecular beams apparatus

This report presents details of the development of a new crossed molecular beams apparatus designed and fabricated to carry out high sensitivity ion translational energy spectrometric investigations of the potential energy surfaces of small molecular species. The translational energy spectrometer is used to carry out experimental studies of ion-neutral reactions resulting in charge stripping of CS+ radicals and dissociation of metastable CO2+ dications. These results are interpreted in the light of high-levelab initia molecular orbital calculations of the pertinent molecular potential energy functions. New results for the double ionisation energy of CS and the kinetic energy released upon dissociation of specific electronic states of CO2+ are presented

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

Double and single ionization of helium by high velocity fully stripped ions

The ratio, R, of the double ionization cross section to that of single ionization for helium by fully stripped carbon, oxygen, silicon and sulphur ions in the energy range of 2.8 to 6.3 MeV amu-1 have been measured. The data obtained using carbon and oxygen projectiles show that in this energy range R is in better agreement with the fit given by Knudsen et al. (1984) than measurements using N7+ projectiles in the energy range 10-30 MeV amu-1 and Kr36+ projectiles with energies 0.5 and 1 GeV amu-1

Dissociative ionization of methane by chirped pulses of intense laser light

Measurements have been made of optical field-induced ionization and fragmentation of methane molecules at laser intensities in the 1016 W cm-2 range using near transform limited pulses of 100 fs duration as well as with chirped pulses whose temporal profiles extend up to 1500 fs. Data is taken both in constant-intensity and constant-energy modes. The temporal profile of the chirped laser pulse is found to affect the morphology of the fragmentation pattern that is measured. Besides, the sign of the chirp also affects the yield of fragments like C2+, H+, and H2+ that originate from methane dications that are formed by optical field-induced double ionization

Strong optical fields induce ultrafast rearrangement of H-atoms in ethanol molecules

H-atoms in C<SUB>2</SUB>H<SUB>5</SUB>OH are rearranged by strong optical fields generated by intense, 100 fs long infrared laser pulses to form new bonds that lead to the H<SUB>3</SUB><SUP>+</SUP> molecular ion. This observation appears to be against the expectation that exposure of molecules to intensities of the order of 10<SUP>15</SUP> W cm<SUP>-2</SUP> inevitably lead to multiple ionization of molecules followed by instantaneous Coulomb explosion into fragments. The polarization dependence of the H<SUB>3</SUB><SUP>+</SUP> signal and of the energy content of H<SUB>3</SUB><SUP>+</SUP> ions lead to believe that H-atom rear-rangement in ethanol occurs within a single 100 fs pulse

Nonadiabatic response of molecules to strong fields of picosecond, femtosecond, and subfemtosecond duration: an experimental study of the methane dication

The double ionization of methane has been accomplished using strong optical fields that are generated using moderately intense lasers, and by strong fields that are induced by fast-moving, highly charged ions. In the former case laser intensities in the range 1014 W cm-2 generate fields whose durations are of 35 ps and 36 fs while in the latter case equivalent fields last for only 200-300 as. The dynamics of the field-ionized electrons are different in the two temporal regimes, fast (picoseconds), and ultrafast (few tens of femtoseconds and subfemtoseconds). Our experiments show that nonadiabatic effects come into play in the ultrafast regime; we directly monitor such effects by measuring the kinetic energy that is released when a specific bond in the doubly charged methane molecular ion breaks

Z-scan studies and optical limiting in a mode-locking dye

The optical nonlinearity of a mode-locking dye (Kodak 9740) under picosecond excitation at 532 nm, is studied using the Z-scan technique. Theoretical fits to the data give a value of 3.2 cm/GW for the two-photon absorption coefficient (&#946;) and -2.1&#215;10-5 cm2/GW for the real part (&#947;) of the nonlinearity. In addition, we demonstrate optical limiting using two-photon absorption as well as self-defocusing