Subfemtosecond control of the breaking and making of chemical bonds in polyatomic molecules is poised to open new pathways for the laser-driven synthesis of chemical products. The break-up of the C-H bond in hydrocarbons is an ubiquitous process during laser-induced dissociation. While the yield of the deprotonation of hydrocarbons has been successfully manipulated in recent studies, full control of the reaction would also require a directional control (that is, which C-H bond is broken). Here, we demonstrate steering of deprotonation from symmetric acetylene molecules on subfemtosecond timescales before the break-up of the molecular dication. On the basis of quantum mechanical calculations, the experimental results are interpreted in terms of a novel subfemtosecond control mechanism involving non-resonant excitation and superposition of vibrational degrees of freedom. This mechanism permits control over the directionality of chemical reactions via vibrational excitation on timescales defined by the subcycle evolution of the laser waveform.

Proton migration is a ubiquitous process in chemical reactions related to biology, combustion, and catalysis. Thus, the ability to manipulate the movement of nuclei with tailored light within a hydrocarbon molecule holds promise for far-reaching applications. Here, we demonstrate the steering of hydrogen migration in simple hydrocarbons, namely, acetylene and allene, using waveform-controlled, few-cycle laser pulses. The rearrangement dynamics is monitored using coincident 3D momentum imaging spectroscopy and described with a widely applicable quantum-dynamical model. Our observations reveal that the underlying control mechanism is due to the manipulation of the phases in a vibrational wave packet by the intense off-resonant laser field. DOI: 10.1103/PhysRevLett.116.193001 The rearrangement of hydrocarbon bonds via the migration of a hydrogen atom can result in major deformations of molecular architecture and, thus, alter the molecule's chemical properties. Examples include keto-enol tautomerism where the migration of a proton changes an aldehyde into an alcohol. Isomerization reactions of that kind have been the subject of numerous studies [1,2]. Of particular interest was to determine the so-called isomerization time, which has been measured to be within several tens of fs in small hydrocarbons [3][4][5][6]. The phenomenon has also been observed in larger molecules, such as protonated triglycine [7]. Tracing of the hydrogen migration from different locations within the molecule has been made possible via isotope labeling; see, e.g., Refs. [8][9][10]. The ability to exert control over the migration could lead to advancement in topics such as the efficiency of catalytic reactions [11] and combustion reactions regarding fuel and energy research [12]. Furthermore, light-induced control of hydrogen migration may open new reaction pathways which cannot materialize by other means.Despite its direct relevance to applied chemistry, studies regarding the control of the hydrogen migration process have been scarce and have been limited to theory [13,14] for a long time. However, recent progress has been made in coherently controlling isomerization reactions using fundamental parameters of ultrafast strong-field laser sources. Xie et al. varied the pulse duration and intensity to explore the isomerization of ethylene [15] and reported control of the total fragmentation yields of various hydrocarbons [16].Here, we demonstrate steering of the direction of the hydrogen migration using the electric field waveform of intense few-cycle laser pulses. This approach goes beyond earlier work on toluene [17] and methanol [18] using twocolor pulses with a duration of tens of fs. In contrast, the duration of our few-cycle laser pulses is significantly shorter than the time scale of the isomerization dynamics, therefore, avoiding charge-resonance-enhanced ionization [19] occurring at large internuclear distances [20]. Moreover, an influence of electron localization-assisted enhanced ionization on the dissociation reactions, recently ...

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...

We reveal surprisingly high kinetic energy release in the intense-field fragmentation of D + 3 to D + + D + + D with 10 16 W cm −2 , 790 nm, 40 fs (and 7 fs) laser pulses. This feature strongly mimics the behaviour of the D + + D + + D + channel. From the experimental evidence, we conclude that the origin of the feature is due to frustrated tunnelling ionization, the first observation of this mechanism in a polyatomic system. Furthermore, we unravel evidence of frustrated tunnelling ionization in dissociation, both two-body breakup to D + D + 2 and D + + D 2 , and three-body breakup to D + + D + D. Gesellschaft processes involving either elastic scattering [6][7][8], inelastic scattering [9, 10], or electron-ion recombination [11]. These phenomena have led to the birth of new areas of research such as high-harmonic generation and attosecond science [12][13][14][15], laser-driven electron diffraction imaging [5][6][7][8], molecular orbital tomography [16,17] and electron wavepacket probing of molecular dynamics [18-21]-naming only a few.Related to the electron recollision process, recently Nubbemeyer et al [22] reported a new phenomenon dubbed frustrated tunnelling ionization (FTI). Demonstrated originally in strong-field ionization of helium, Nubbemeyer et al showed that an electron wavepacket that starts to tunnel away from the core in an intense laser field, but fails to acquire sufficient drift momentum to escape the attractive potential of the remaining He + ion, can be captured into an excited Rydberg orbital of the He atom-in effect 'frustrating' the tunnel ionization process. This process must occur during the laser pulse to conserve energy and momentum, most likely during the trailing edge, as the electron is gently decelerated over many laser cycles before being pulled into orbit.The same mechanism has been observed in the dissociative ionization of a few diatomic molecules (H 2 [23], D 2 [24], O 2 [25] and Ar 2 [26][27][28]). For such molecules, following ionization, an electron that is excited to the continuum and driven by the laser field tends to be captured to a Rydberg orbital of one of the two 'Coulomb-exploding' fragment ions. The signature of frustrated tunnelling in molecules is that, counterintuitively, the final kinetic energy release (KER) is similar to that of a Coulomb explosion event even though only one product fragment is charged while the other fragment is neutral [23].This description of FTI uses language, such as electron capture, that is usually reserved for discussions involving ionization. Throughout the paper we use this language for convenience. However, it does pose an interesting question in relation to the actual mechanism for FTI, that is,