Intense Two-Cycle Laser Pulses Induce Time-Dependent Bond Hardening in a Polyatomic Molecule

Abstract

A time-dependent bond-hardening process is discovered in a polyatomic molecule (tetramethyl silane, TMS) using few-cycle pulses of intense 800 nm light. In conventional mass spectrometry, symmetrical molecules like TMS do not exhibit a prominent molecular ion (TMS + ) as unimolecular dissociation into [Si(CH 3 ) 3 ] + proceeds very fast. Under strong field and few-cycle conditions, this dissociation channel is defeated by time-dependent bond-hardening: a field-induced potential well is created in the TMS + potential energy curve that effectively traps a wavepacket. The timedependence of this bond hardening process is verified using longer-duration (≥ 100 fs) pulses; the relatively "slower" fall-off of optical field in such pulses allows the initially trapped wavepacket to leak out, thereby rendering TMS + unstable once again. Our results are significant as they demonstrate (i) optical generation of polyatomic ions that are normally inaccessible and (ii) optical control of dynamics in strong fields, with distinct advantages over weak-field control scenarios that demand a narrow bandwidth appropriate for a specified transition.

Preface- Ultrafast Science: Progress and Opportunities

“…Such rearrangement is the precursor of H 2 + in a well-defined spatial region within the PES of the dication. The norm of the wave packet entering this zone was deduced (Garg et al, 2012) to be about 1.4% for the O+H 2 + fragmentation channel, in remarkable consonance with the experimental yield (Fig. 1).…”

“…Subsequent time-dependent quantum dynamics of this dication have offered support for the experimental findings shown in Fig. 1 (Dharmadhikari et al, 2009;Garg et al, 2012). Potential energy surfaces (PES) of the ground electronic state of H 2 O 2+ were generated by solving the three-dimensional time dependent Schrödinger equation in Jacobi coordinates for an initial wavepacket (which is the ground vibrational state wave function of neutral H 2 O) whose time evolution was followed for many small-time steps on the dication PES.…”

“…This results in an effective hardening of the bond. The time dependence of this bond-hardening process has been verified using longer-duration (40 fs, 100 fs) pulses and it has been demonstrated that the relatively slower fall-off of optical field in these longer pulses enables the initially trapped wave packet to leak out, thereby rendering TMS + unstable once again (Dota et al, 2012).…”

“…Conventional mass spectrometry has long established that ionization of symmetrical polyatmics like TMS does not yield a molecular ion (TMS + ) as unimolecular dissociation into TMS-CH 3 proceeds (Dota et al, 2012) that this dissociation channel is circumvented because of a time-dependent bond hardening process wherein a field-induced potential well is created in the TMS + potential energy surface that effectively traps a wave packet. This results in an effective hardening of the bond.…”

Strong-field approximation for above-threshold ionization of polyatomic molecules

“…For example, recent developments in this field led to experiments on manipulating chemical reactions [6], tomographic imaging of molecular orbitals [7], and probing the nuclear dynamics on the attosecond and femtosecond time scales [8,9]. Furthermore, molecules, exposed to a strong laser field, exhibit effects such as molecular alignment due to the laser field [10][11][12], dissociation [13], coherent control of the molecular orientation [14][15][16], Coulomb explosion [17][18][19], and time-dependent bond hardening in polyatomic molecules [20].…”