Carrier-Envelope Phase Control over Pathway Interference in Strong-Field Dissociation ofH2+

Abstract

The dissociation of an H + 2 molecular-ion beam by linearly polarized, carrier-envelope-phase-tagged 5 fs pulses at 4×10 14 W/cm 2 with a central wavelength of 730 nm was studied using a coincidence 3D momentum imaging technique. Carrier-envelope-phase-dependent asymmetries in the emission direction of H + fragments relative to the laser polarization were observed. These asymmetries are caused by interference of odd and even photon number pathways, where net-zero photon and 1-photon interference predominantly contributes at H + +H kinetic energy releases of 0.2 -0.45 eV, and net-2-photon and 1-photon interference contributes at 1.65 -1.9 eV. These measurements of the benchmark H + 2 molecule offer the distinct advantage that they can be quantitatively compared with ab initio theory to confirm our understanding of strong-field coherent control via the carrier-envelope phase. PACS numbers: XXXOne ultimate goal of ultrafast, strong-field laser science is to coherently control chemical reactions [1][2][3]. A prerequisite to achieving this goal is to understand the control mechanisms and reaction pathways. To this end, tailoring the electric field waveform of few-cycle laser pulses to control reactions and uncover the underlying physics has become a powerful tool [4][5][6]. It has been applied to the dissociative ionization of H 2 and its isotopologues [7][8][9][10][11][12] and has recently been extended to more complex diatomic molecules, such as CO [13][14][15], and to small polyatomic molecules [16,17].Conceptually, one of the most basic features of a fewcycle laser pulse to control is the carrier-envelope phase (CEP). When the laser's electric field is written as E(t) = E 0 (t) cos(ωt + φ), E 0 (t) is an envelope function, ω is the carrier angular frequency, and φ is the CEP. In fact, all of the few-cycle waveform experiments cited above used the CEP as the control parameter.For example, Kling et al. used 5 fs, 1.2×10 14 W/cm 2 pulses with stabilized CEP to dissociatively ionize D 2 and found asymmetries in the emission direction of D + ions for kinetic energy releases (KER) above 6 eV [7,8]. The diminished dissociation signal in a circularly polarized laser field indicated that recollision played a role. Recollision entails a tunnel-ionized electron undergoing a collision with its parent ion after acceleration by the oscillating laser field [18,19]. The energy exchange between the laser-driven electron and the parent ion can promote the D + 2 to the 2pσ u excited state. Coupling of the 2pσ u and 1sσ g states [20] on the trailing edge of the laser pulse during the dissociation of D + 2 was suggested as the explanation for the CEP-dependent asymmetry [7,8].Another example comes from Kremer et al. who exposed an H 2 target to 6 fs, 4.4×10 14 W/cm 2 CEPstabilized laser pulses and observed asymmetries for KER values between 0.4 and 3 eV [9] -energies they attributed to bond softening (BS) [21] and not electron recollision, which has higher KER. They proposed that the initial ionization of H 2 generates a coherent wav...

Phase- and intensity-dependence of ultrafast dynamics in hydrocarbon molecules in few-cycle laser fields

“…A large part of previous work was performed on molecular hydrogen and molecular hydrogen ions (see e.g. [8][9][10][11][12][13][14][15][16][17][18]). …”

Effect of nuclear mass on carrier-envelope-phase-controlled electron localization in dissociating molecules

“…In our own experiments on H 2 with even more intense pulses (6 × 10 14 W/cm 2 ), we observed a very high CEP-dependent asymmetry of 40%, as well as modulation of up to 5% in dissociation yield [3]. More recently, studies of CEP-dependent dissociation using an H 2 + ion beam have also been reported [8,9]. These experiments used 4.5 [8] and 5 fs [9] pulses of 4 × 10 14 W/cm 2 peak intensity and also measured significant asymmetry, up to 30% for [8], in the low-KER channels.…”

Spatial and temporal control of populations, branching ratios, and electronic coherences in LiH by a single one-cycle infrared pulse

“…Several experimental schemes rely on the CEP phase value of a CEP stable few cycle pulse [17] for tailoring the electric field profile of the pump pulse and controlling electron localization and dissociation. [8,[18][19][20][21][22][23][24][25] Photoinduced coherent electronic dynamics can also be probed using high harmonic generation [13,[26][27][28], transient absorption spectroscopy [29][30][31], time-resolved molecular-frame photoelectron-angular-distribution [32] and attoclock measurements [33,34].…”