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We report on the observation of resonance enhanced high-order harmonic generation (HHG) from a dense medium composed of indium or tin nanoparticles. Unlike previous studies of resonance enhanced high harmonics from laser produced plasma plume from a prepulse, we generate HHG directly, thus eliminating the undesired contributions from various components in the plasma. We discuss the mechanisms for our observed resonance effect in the absence of preionized particles and the implications of the experimental results on the HHG mechanism in metal nanoparticles.

In this work we perform a polarization dependence study of enhanced ionization (EI) in diatomic molecules. We find that EI exists when the field polarization is parallel to the molecular axis but disappears when polarization is perpendicular. We further study EI with circular polarization and find that EI exists with circular polarization indicating that rescattering does not play a significant role for EI. Furthermore, we study molecular orbital effect on EI. We find that EI exists in \ensuremath{\sigma} type but not \ensuremath{\pi} type outmost molecular orbitals.

In this paper we study double-ionization-induced charge asymmetric dissociation (CAD) in heteronuclear diatomic molecules. In CO we find a selective charge distribution in two CAD channels, i.e., ${\mathrm{C}}^{2+}+\phantom{\rule{1.0pt}{0ex}}\mathrm{O}$ is abundantly produced but $\mathrm{C}+{\mathrm{O}}^{2+}$ is nearly nonexistent. This cannot be explained by the ionization energy difference between the two channels alone. Our study shows that the ${\mathrm{C}}^{2+}+\mathrm{O}$ channel is sequentially formed through an intermediate state ${\mathrm{C}}^{+}+\mathrm{O}$ and the selective charge distribution is the result of electron distribution in CO when exposed to intense laser fields.

Enhanced ionization (EI) of molecules has been extensively studied over the past two decades as a common process in molecular dissociative ionization in strong laser fields. Direct evidence for EI has been found only in ${\mathrm{I}}_{2}$ and ${\mathrm{H}}_{2}$. However, in this work we perform a direct study of EI in CO and ${\mathrm{N}}_{2}$, and find enhanced ionization in an alternate dissociation channel in each of these two molecules following double ionization. Surprisingly, EI does not happen in the commonly seen dissociation channels that were previously assigned undergoing EI. Instead, EI occurs only in the alternate channels seen here with a lower kinetic-energy release.

Strong-field single ionization and double ionization of two diatomic molecules, ${\mathrm{O}}_{2}$ and ${\mathrm{N}}_{2},$ are studied and compared to Xe and Ar, using an intense ultrashort pulse Ti:sapphire laser in the $2\ifmmode\times\else\texttimes\fi{}{10}^{13}$ to $8\ifmmode\times\else\texttimes\fi{}{10}^{14}{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$ intensity range. ${\mathrm{N}}_{2}$ behaves like a structureless atom for both single and double ionization. The recently reported suppression of the ${\mathrm{O}}_{2}^{+}$ ion yield compared to ${\mathrm{Xe}}^{+}$ is confirmed in our experiment, but we show that the suppression is not due to dissociative recombination. Rather, we conclude that the ionization rate of ${\mathrm{O}}_{2}$ is below that predicted by tunneling ionization. We extend the study to the double ionization of ${\mathrm{O}}_{2}$ and find a distinctly reduced nonsequential double-ionization rate. We find evidence that electronic structure influences strong-field tunneling ionization in molecules.

We present ion time-of-flight data showing strong-field ionization of iodine molecules up to ${\mathrm{I}}_{2}^{13+}$ using 33-fsec laser pulses. Every even molecular charge state shows an charge asymmetric dissociation channel $({\mathrm{I}}_{2}^{2n+}\ensuremath{\rightarrow}{\mathrm{I}}^{(n+1)+}+{\mathrm{I}}^{(n\ensuremath{-}1)+})$ with a significant branching ratio (15--30 %) in addition to the symmetric dissociation channel $({\mathrm{I}}_{2}^{2n+}\ensuremath{\rightarrow}{\mathrm{I}}^{n+}+{\mathrm{I}}^{n+})$ showing the general nature of this process. Furthermore, all charge states are formed at internuclear separations less than the critical separation for electron localization and enhanced ionization ${(R}_{c}).$ One channel, in particular, ${\mathrm{I}}_{2}^{10+}\ensuremath{\rightarrow}{\mathrm{I}}^{6+}+{\mathrm{I}}^{4+},$ represents a direct electronic excitation by nonresonant strong-field interaction to a state 20.9 eV above the ground state $({\mathrm{I}}_{2}^{10+}\ensuremath{\rightarrow}{\mathrm{I}}^{5+}+{\mathrm{I}}^{5+}).$ We show that the interaction of the dipole moment of the asymmetric charge distribution with the external field can account for this energy and the excitation mostly likely involves charge-transfer states. These results establish strong-field ionization of molecules as an efficient method for populating highly excited electronic states.

In this paper, we study double- and triple-ionization-induced dissociation of N{sub 2}. With an improved temporal resolution of distinguishing neighboring channels in time-of-flight spectra, we observe two N{sup +} + N{sup +} states, a fast channel with a higher kinetic-energy release (KER) and a slow channel with a lower KER. The slow KER channel has not been seen in previous work. We also study triple-ionization-induced dissociation channel N{sup 2+} + N{sup +} by analyzing its intensity-dependent KER. Our study shows that the N{sup 2+} + N{sup +} channel is predominantly formed nonvertically through the low KER dissociation N{sup +} + N{sup +} state.

In this paper, we extend strong-field double ionization from diatomic molecules to triatomic molecules. Strong-field double ionization of CO${}_{2}$ is studied here with both linear and circular polarizations. A clear nonsequential signature is seen in doubly ionized CO${}_{2}$. We also compare nonsequential double ionization behaviors of CO${}_{2}$ to those of the well-studied diatomic molecules, and the difference is attributed to the detailed electronic structures in molecules.

In this paper, we study nonsequential double ionization in various molecular fragments for laser polarization aligning either parallel or perpendicular to the molecular axis. In the diatomic molecule ${\mathrm{O}}_{2}$, we find that nonsequential double ionization can be turned on and off when we simply switch the laser polarization between perpendicular and parallel ionization. Furthermore, we find that the dependence of kinetic energy release on intensity can be used to distinguish sequential versus nonsequential ionization in molecules. The results in this paper allow us to gain better understanding of the role of nonsequential ionization in molecular dissociative ionization.

In this paper, we study triple-ionization-induced dissociation in ${\mathrm{N}}_{2}$ and ${\mathrm{O}}_{2}$ for laser polarization aligning either parallel or perpendicular along the molecular axis. For the first time, we demonstrate that vertical or nonvertical transition can be turned on and off by simply switching the laser polarization between being perpendicular and parallel to the molecular axis. We also show that the dependence of kinetic energy release on intensity can be used to distinguish vertical from nonvertical transition in triple-ionization-induced dissociation.

In this paper, we study the dynamics of triple-ionization-induced dissociation in a heteronuclear diatomic molecule $\mathrm{NO}$. Compared to homonuclear diatomic molecules, $\mathrm{NO}$ shows a greater complexity in its final states following triple ionization. By utilizing a well-established technique from the study of sequential versus nonsequential ionization, our study shows that both the ${\mathrm{N}}^{2+}+{\mathrm{O}}^{+}$ and ${\mathrm{N}}^{+}+{\mathrm{O}}^{2+}$ channels are predominately formed nonvertically through a relatively slowly dissociating ${\mathrm{N}}^{+}+{\mathrm{O}}^{+}$ state. Finally, we show that both the ${\mathrm{N}}^{2+}+{\mathrm{O}}^{+}$ and ${\mathrm{N}}^{+}+{\mathrm{O}}^{2+}$ channels are formed at nearly the same internuclear separation that is much smaller than the critical internuclear distance, indicating that dissociative ionization of high charge states in a molecule can occur as a molecule steadily expands from its equilibrium separation rather than always at the critical internuclear distance.

In this paper, we report the first comparison study between ${\mathrm{N}}_{2}$ and ${\mathrm{O}}_{2}$ on their triple-ionization-induced dissociation channels using near-IR 800-nm ultrashort laser pulses. Our experiment shows that the ${\mathrm{O}}^{2+}+{\mathrm{O}}^{+}$ channel is predominately formed nonvertically through the intermediate ${\mathrm{O}}^{+}+{\mathrm{O}}^{+}$ channel, while the ${\mathrm{N}}^{2+}+{\mathrm{N}}^{+}$ channel appears to be formed vertically at the lower intensity range but nonvertically at higher intensities. Interestingly, ${\mathrm{N}}^{2+}+{\mathrm{N}}^{+}$ appears to be in the electronically excited state when it is reached vertically in the lower intensity range. The different triple ionization and dissociation behaviors between ${\mathrm{N}}_{2}$ and ${\mathrm{O}}_{2}$ can be understood by considering the ionization dynamics of the two molecules in our ultrashort laser pulses.

Metastable channel of doubly ionized carbon monoxide, ${\mathrm{CO}}^{2+}$, was scantly seen in previous strong-field experiments at the visible wavelength region, but was commonly observed using single high-energy photon or electron excitation. For the first time with near-IR ultrashort-pulse radiation, we observe an abundance of ${\mathrm{CO}}^{2+}$. We show that ${\mathrm{CO}}^{2+}$ results from nonsequential double ionization, while its dissociation counterpart, ${\mathrm{C}}^{+}+{\mathrm{O}}^{+}$, results from sequential processes, and ${\mathrm{CO}}^{2+}$ can be obtained through either single high-energy photon or electron excitation or multiphoton ionization with ultrashort pulses before a critical internuclear distance is reached. Our study demonstrates the experimental conditions to converge the outcomes from two vastly different regimes, namely, multiphoton excitation and ionization in strong fields and single high-energy photon or electron excitation and ionization in weak fields.

In this paper, we report on the observation of channel competition following double ionization of NO molecules in femtosecond strong laser fields. We show that slight variations of two parameters of laser light, intensity and polarization, lead to dramatic change in the branching ratio between the metastable and dissociation channels of doubly ionized NO molecules. The difference of electronic configuration between the metastable and dissociation channels indicates that our observation results from the competing processes of sequential and nonsequential double ionization of NO. This study suggests that multielectron collective effects may be utilized in obtaining a desirable final state among several competing channels.

In this paper, we report on an apparently anomalous observation in the dissociation channels of singly ionized nitric oxide (NO) molecules. We observe that the NO molecules, when oriented perpendicular to the laser polarization, appear to preferentially dissociate into the ${\mathrm{N}}^{+}+\mathrm{O}$ channel rather than the $\mathrm{N}+{\mathrm{O}}^{+}$ channel, despite the fact that the latter channel is more energetically favorable. A careful examination of the unique electronic structure of the NO molecule suggests that this observation is most likely due to the influence of the detailed electronic structure of NO on its strong-field ionization dynamics, which leads $\mathrm{N}{\mathrm{O}}^{+}$ created in strong fields to dissociate differently from that in weak fields. This study reinforces our understanding of the fundamental differences between ionization dynamics in strong fields versus weak fields.

By studying high-precision ion yield measurements in diatomic molecules ${\mathrm{N}}_{2}$ and NO with 30-fs linearly (LP) and circularly (CP) polarized light, we extend to molecular systems the study of ellipticity effects on both single and double ionization in strong laser fields. Competing ionization rates between LP and CP light, as predicted in the Ammosov-Delone-Krainov model, is observed experimentally by studying singly ionized ${\mathrm{N}}_{2}$ and NO. We further observe an anomalously high ${\mathrm{NO}}^{2+}$ signal with CP, which shows a nonsequential ionization signature. A hypothesis is given that ties this observation to the influences of electronic structure on ellipticity effects in strong-field ionization.