Steering Proton Migration in Hydrocarbons Using Intense Few-Cycle Laser Fields

Abstract

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 ...

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

“…[32] for the deprotonation of acetylene and in Ref. [34] for the isomerization of acetylene. Briefly, the wave packet possesses a preferential propagation direction, which is determined by the CEP.…”

“…[30][31][32][33][34][35]). CEP effects in the deprotonation of acetylene, induced by few-cycle laser pulses, have attracted considerable interest.…”

Time-resolved nuclear dynamics in bound and dissociating acetylene

“…Upon ionization, the molecular structure and the corresponding chemical properties can change drastically, e.g., due to proton migration, 1 dissociation, 2 or even selective bond-breaking 3 . To temporally resolve the resultant vibrational motion, Coulomb explosion imaging (CEI) can be applied to reveal the underlying nuclear dynamics.…”

Coherent vibrations in methanol cation probed by periodic H3+ ejection after double ionization

“…It has also been shown that H 3 + can be emitted from hydrocarbon molecular dication prepared by electron impact ionization 2 , by extreme ultraviolet photoionization 3 , and by irradiation of intense femtosecond laser pulses [4][5][6] . The formation of H 3 + from allene [7][8][9] and the formation of HD 2 + from CD 3 OH 10 showed that migration of hydrogen atoms can proceed in the formation of H 3 + . Recently, we interpreted the mechanism of H 3 + formation from dication of methanol theoretically as a unimolecular decomposition process in which a long-lived neutral moiety of H 2 , formed within a dication molecule, abstracts a proton in the other moiety having the charge of +2 11 .…”