Softening of theH2+molecular bond in intense laser fields

Abstract

We report a new kind of multiphoton dissociation in intense laser fields. H2 + molecular ions were formed in above-threshold ionization of H2 gas by intense 532-nm, 100-ps laser pulses. Observations suggest that multiphoton couplings soften the molecular bond, resulting in dissociation at laser intensities over 50 TW/cm 2 . The ion-dissociation spectra have multiple peaks caused by multiphoton transitions during dissociation of Hh + . PACS numbers: 33.80.WzInterest in small diatomic molecules irradiated by intense laser light has been stimulated by high-intensity photoionization experiments on atoms. At intensities above 10 TW/cm 2 atoms exhibit "above-threshold" ionization (ATI), where they absorb more photons than the minimum needed to ionize. 1,2 Molecules also exhibit ATI, but have additional degrees of freedom that lead to other new phenomena as well, such as multiphoton dissociation and dissociative ionization. 3 " 6 Intense laser fields can alter the kinetic energies of molecular fragments, through Coulomb repulsion following rapid multiple ionization. 7 " 9In this Letter, we report a new phenomenon in intense fields: the softening of molecular bonds. For this study we have chosen nature's simplest molecule, the H2 + molecular ion. We find that in laser fields comparable to the internuclear binding fields (« 3 V/A, or peak intensities of ~ 100 TW/cm 2 ) molecules can become unstable. In effect, the internuclear potentials become repulsive. However, in contrast to the "Coulomb explosions" described above, where dissociation is caused by sudden removal of electrons, the strength of the repulsion due to bond softening is quite gentle, and does not involve removal of any electrons at all: Molecular fragments usually emerge with kinetic energy equivalent to less than one photon.This new phenomenon is similar to ordinary photodissociation, where a molecule falls apart following excitation to a predissociating electronic state. The highintensity version of the process differs, however, in that it involves both multiphoton absorption and stimulated emission. Many photons are absorbed, but most of their energy is returned to the photon field via stimulated emission during the dissociation, resulting in slow ion fragments. The suggestion that molecules might deform in this manner was proposed previously by others. 10 " 12 This is the first experimental verification of this phenomenon.These experiments employed an amplified and frequency-doubled (A, =532 nm, ft<w=2.33 eV), modelocked g-switched Nd-doped yttrium-aluminum-garnet laser, producing 25-mJ pulses between 70 and 100 ps in

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].…”

Molecular events in the light of strong fields: A light‐induced potential scenario

“…The effect of a strong field is to mix their electronic character. The first evidence of these properties was found out after observing bond softening in the ground electronic state [72][73][74][75][76][77] and bond hardening in a dissociative potential. [78][79][80][81][82] Varying the intensity and frequency of the dressing field one can efficiently control the geometry of the "previously-dissociating" potential.…”

Time-resolved Coulomb-explosion imaging of nuclear wave-packet dynamics induced in diatomic molecules by intense few-cycle laser pulses

“…Four more spectra for the nuclear wave-packet propagation on which agrees very well with the corresponding features in the experimental spectra. Figure 18 shows the time-dependent KER spectra for four final breakup channels: (1,2), and C 2+ + O 2+ (2,2). For the (1,1) channel [ Fig.…”

“…In particular, such phenomena as bond softening [1][2][3][4] and charge-resonance enhanced ionization [5] (CREI) were first discovered, studied, and understood in hydrogen. Hydrogen was also the molecule in the seminal "molecular clock" experiments [6,7].…”