Roaming mechanisms, involving the brief generation of a neutral atom or molecule that stays in the vicinity before reacting with the remaining atoms of the precursor, are providing valuable insights into previously unexplained chemical reactions. Here, the mechanistic details and femtosecond time-resolved dynamics of H3+ formation from a series of alcohols with varying primary carbon chain lengths are obtained through a combination of strong-field laser excitation studies and ab initio molecular dynamics calculations. For small alcohols, four distinct pathways involving hydrogen migration and H2 roaming prior to H3+ formation are uncovered. Despite the increased number of hydrogens and possible combinations leading to H3+ formation, the yield decreases as the carbon chain length increases. The fundamental mechanistic findings presented here explore the formation of H3+, the most important ion in interstellar chemistry, through H2 roaming occurring in ionic species.

Strong-field laser-matter interactions often lead to exotic chemical reactions. Trihydrogen cation formation from organic molecules is one such case that requires multiple bonds to break and form. We present evidence for the existence of two different reaction pathways for H3 + formation from organic molecules irradiated by a strong-field laser. Assignment of the two pathways was accomplished through analysis of femtosecond time-resolved strong-field ionization and photoion-photoion coincidence measurements carried out on methanol isotopomers, ethylene glycol, and acetone. Ab initio molecular dynamics simulations suggest the formation occurs via two steps: the initial formation of a neutral hydrogen molecule, followed by the abstraction of a proton from the remaining CHOH2+ fragment by the roaming H2 molecule. This reaction has similarities to the H2 + H2 + mechanism leading to formation of H3 + in the universe. These exotic chemical reaction mechanisms, involving roaming H2 molecules, are found to occur in the ~100 fs timescale. Roaming molecule reactions may help to explain unlikely chemical processes, involving dissociation and formation of multiple chemical bonds, occurring under strong laser fields.

A key question concerning the three-body fragmentation of polyatomic molecules is the distinction of sequential and concerted mechanisms, i.e., the stepwise or simultaneous cleavage of bonds. Using laser-driven fragmentation of OCS into O^{+}+C^{+}+S^{+} and employing coincidence momentum imaging, we demonstrate a novel method that enables the clear separation of sequential and concerted breakup. The separation is accomplished by analyzing the three-body fragmentation in the native frame associated with each step and taking advantage of the rotation of the intermediate molecular fragment, CO^{2+} or CS^{2+}, before its unimolecular dissociation. This native-frame method works for any projectile (electrons, ions, or photons), provides details on each step of the sequential breakup, and enables the retrieval of the relevant spectra for sequential and concerted breakup separately. Specifically, this allows the determination of the branching ratio of all these processes in OCS^{3+} breakup. Moreover, we find that the first step of sequential breakup is tightly aligned along the laser polarization and identify the likely electronic states of the intermediate dication that undergo unimolecular dissociation in the second step. Finally, the separated concerted breakup spectra show clearly that the central carbon atom is preferentially ejected perpendicular to the laser field.

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...

Shaping ultrafast laser pulses using adaptive feedback can manipulate dynamics in molecular systems, but extracting information from the optimized pulse remains difficult. Experimental time constraints often limit feedback to a single observable, complicating efforts to decipher the underlying mechanisms and parameterize the search process. Here we show, using two strong-field examples, that by rapidly inverting velocity map images of ions to recover the three-dimensional photofragment momentum distribution and incorporating that feedback into the control loop, the specificity of the control objective is markedly increased. First, the complex angular distribution of fragment ions from the no þis controlled via a barrier-suppression mechanism, a result that is validated by model calculations. Collectively, these experiments comprise a significant advance towards the fundamental goal of actively guiding population to a specified quantum state of a molecule.

A comparative study of bond rearrangement is reported for the double ionization of three triatomic molecules: carbon dioxide, carbonyl sulfide, and water (D2O). Specifically we study the formation of the molecular cation AC + from the edge atoms of a triatomic molecular dication ABC 2+ following double ionization by intense, short (23 fs, 790 nm) laser pulses. The comparison is made using the double ionization branching ratio of each molecule, thereby minimizing differences due to differing ionization rates. The rearrangement branching ratio is highest for water, which has a bent initial geometry, while CO2 and OCS are linear molecules. The angular distribution of O + 2 fragments arising from CO2 is essentially isotropic, while SO + from OCS and D + 2 from D2O are aligned with the laser polarization. In the CO2 and D2O cases, the angular distributions of the bond rearrangement channels are different from the angular distributions of the dominant dissociative double ionization channels CO + + O + and OD + + D + . Only the angular distribution of SO + from OCS is both aligned with the laser polarization and similar to the angular distribution of the largest dissociative channel, CO + + S + . The mixed behavior observed from the angular distributions of the different molecules stands in contrast to the relative consistency of the magnitude of the bond rearrangement branching ratio.I.

Ultrafast science depends on different implementations of the well-known pump–probe method. Here, we provide a formal description of ultrafast disruptive probing, a method in which the probe pulse disrupts a transient species that may be a metastable ion or a transient state of matter. Disruptive probing has the advantage of allowing for simultaneous tracking of the yield of tens of different processes. Our presentation includes a numerical model and experimental data on multiple products resulting from the strong-field ionization of two different molecules, partially deuterated methanol and norbornene. The correlated enhancement and depletion signals between all the different fragmentation channels offer comprehensive information on photochemical reaction pathways. In combination with ion imaging and/or coincidence momentum imaging or as complementary to atom-specific probing or ultrafast diffraction methods, disruptive probing is a particularly powerful tool for the study of strong-field laser–matter interactions.

Using the CD3OH isotopologue of methanol, the ratio of D2H+ to D3+ formation is manipulated by changing the characteristics of the intense femtosecond laser pulse. Detection of D2H+ indicates a formation process involving two hydrogen atoms from the methyl side of the molecule and a proton from the hydroxyl side, while detection of D3+ indicates local formation involving only the methyl group. Both mechanisms are thought to involve a neutral D2 moiety. An adaptive control strategy that employs image-based feedback to guide the learning algorithm results in an enhancement of the D2H+/D3+ ratio by a factor of approximately two. The optimized pulses have secondary structures 110–210 fs after the main pulse and result in photofragments that have different kinetic energy release distributions than those produced from near transform limited pulses. Systematic changes to the linear chirp and higher order dispersion terms of the laser pulse are compared to the results obtained with the optimized pulse shapes.