Manipulation of Bond Hardening inH2+by Chirping of Intense Femtosecond Laser Pulses

Abstract

Bond hardening of H 1 2 has been observed in the intensity range of 100 200 TW͞cm 2 using 792 nm laser pulses. This effect can be understood in terms of a light-induced potential well created at twice the normal (free) equilibrium internuclear distance by an adiabatic mixing of 1-and 3-photon resonances. The trapped population dissociates into H 1 and H when the potential well becomes convex on the trailing edge of the pulse. The dynamics of the nuclear wave packet was manipulated by chirping the pulse duration from 45 to 500 fs and observing a reduction of the kinetic energy release from 0.3 to 0.0 eV. This energy shift is interpreted as a dynamic Raman effect within the laser bandwidth. resonances were inferred from rather noisy data in the early 1990's. They were received with great interest, as at that time the stabilization of the molecular bond was a candidate for a universal mechanism explaining the invariance of ion kinetic energies with changes of intensity and pulse duration [5,6]. Later, it was established that this invariance is a signature of rapid, sequential ionization at the critical internuclear distance [7][8][9]. Since then there has been a surprising lack of clearcut confirmation of the bond hardening effect. With more recent work casting doubt on the existence of light-induced bound states [10,11], the idea of bond hardening has become again only a remote theoretical possibility. Against this trend of scepticism, we present an experimental observation of bond hardening in H 1 2 . In these experiments, chirped pulses from a Ti:sapphire laser were amplified to 10 mJ in energy and compressed to about 50 fs duration at a repetition rate of 10 Hz (see [12] for a recent review of high power ultrafast lasers). The pulse bandwidth had an almost perfect Gaussian shape centered at 792 nm and an extent of 22 nm, full width at half maximum (FWHM). The pulse length was varied by scanning the separation of the two compressor gratings and introducing some uncompensated chirp. The linearly polarized beam, 5 mm in diameter, was focused in an ultrahigh vacuum chamber using an f͞4 parabolic mirror to give a peak intensity of the order of 10 14 W͞cm 2 . Hydrogen gas was introduced into the vacuum chamber via simple effusion, raising the ambient pressure to about 10 26 torr. At this pressure and intensity no space charge effects were observed. Following the process of multiphoton ionization of H 2 , an external electric field directed "forward" and "backward" fragment ions into a vertical, 13-cm-long drift tube. Ions were detected by microchannel plates with a 10-mm circular restriction in front to improve the energy and angular resolutions. The ion time-of-flight (TOF) spectrum and pulse energy were recorded at each laser shot by a digital oscilloscope and stored in a computer. Figure 1 shows ion TOF spectra recorded at several pulse lengths. For each grating separation the peak intensity was adjusted to about 150 TW͞cm 2 , i.e., below saturation of any ion channel [13]. At each laser pulse the ion signal and t...

A framework for understanding molecular ionization in strong laser fields

“…In both cases, the strong gerade-ungerade (g ↔ u) coupling in the potential plays an important role [12]. This coupling also gives rise to the phenomena of bond softening and hardening [13,[31][32][33][34]. In our data, ionization from (1, 2) → (2, 2) and (2, 3) → (3, 3) falls under this category.…”

Molecular resolvent-operator method: Electronic and nuclear dynamics in strong-field ionization

“…Several new concepts have arisen from the understanding of molecular dissociation and ionization in strong fields, such as bond softening [1], bond hardening [2], or charge-resonance-enhanced ionization (CREI) [3]. In particular, the hydrogen molecule and its molecular ion, due to their simplicity, have been the subject of many theoretical and experimental works over the years [4][5][6].…”

Counterintuitive Alignment ofH2+in Intense Femtosecond Laser Fields

“…Dissociation of H 2 1 via one-photon absorption gives one "forward" and one "backward" H 1 peak, 1v, with about 3 eV kinetic energy release (KER). The near-zero KER peak involves zero-photon dissociation (ZPD) [13], where no net number of photons is absorbed, despite energy being extracted from the laser field in a dynamic Raman process [11,12].The variation of the detector angular acceptance with the KER is plotted above the H 1 peaks. We see that the ZPD protons are collected from the full solid angle, and this explains the lack of variability of this peak with the polarization angle.…”