Electron Localization in Molecular Fragmentation ofH2by Carrier-Envelope Phase Stabilized Laser Pulses

Abstract

Fully differential data for H2 dissociation in ultrashort (6 fs, 760 nm), linearly polarized, intense (0.44 PW/cm{2}) laser pulses with a stabilized carrier-envelope phase (CEP) were recorded with a reaction microscope. Depending on the CEP, the molecular orientation, and the kinetic energy release (KER), we find asymmetric proton emission at low KERs (0-3 eV), basically predicted by Roudnev and Esry, and much stronger than reported by Kling et al. Wave packet propagation calculations reproduce the salient features and discard, together with the observed KER-independent electron asymmetry, the first ionization step to be the reason for the asymmetric proton emission.

Understanding the role of phase in chemical bond breaking with coincidence angular streaking

“…2c) for E k 40.6 eV. This confirms that our measured asymmetry is indeed due to interference between the 1o and net-2o dissociating wavepackets [20][21][22][23][24][25][26][27][28][29][30]32 . In our second two-state quantum mechanical model, we numerically solved the one-dimensional time-dependent Schrödinger equation for the vibrational nuclear wavepacket 33 …”

“…Electron localization in the breakup of hydrogen molecular ions has previously been observed [20][21][22][23][24] with symmetry-broken laser fields produced by CEP stabilizing a few-cycle pulse [20][21][22] or by composing a pulse of two different carrier frequencies 23,24 . The asymmetric fields drive and eventually localize the bound electron at one of the dissociating nuclei 25 .…”

“…The kinetic energy spectrum shifts when f el mol is changed by 180°. This is indicative of a kinetic-energy-dependent asymmetric dissociation or electron localization due to the interference between the 1o-and net-2o-dissociating wavepackets [20][21][22][23][24][25][26][27][28][29][30]32 .…”

“…A vibrational nuclear wavepacket on the 1ss g þ curve is created by the early part of the laser pulse at time t i . The wavepacket is coupled forth-and-back between the 2ps u þ and 1ss g þ states at later times (t c ) when the energy gap between the potential curves matches the photon energy (or three times the photon energy), with the molecular ion eventually dissociating into H þ þ H. Because of the coherent superposition of 1ss g þ and 2ps u þ contributions [20][21][22][23][24][25][26][27][28][29][30] , the electron probability density localizes asymmetrically at the nuclei. This electron localization dynamics is governed by the laser phase at the instant of field ionization (t i ) and by the coupling times t c,3o , t c,1o and t 0 c,1o , which depend on the chosen photon energy and on the shape of the potential energy curves involved.…”

“…The data show that electron localization is not something that has to be artificially enforced by optical means [20][21][22][23][24][25][26][27][28] , but occurs naturally, even in pulses that are perfectly symmetric. Our technique, involving the coincidence detection of a streaked electron with a fragment ion, enables the phase and energy dependence of electron localization along a molecular bond to be explored in a transparent way.…”