Ultrafast disruptive probing: Simultaneously keeping track of tens of reaction pathways

Abstract

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.

Coulomb Explosion Dynamics of Multiply Charged para-Nitrotoluene Cations

“…Ultrafast disruptive probing was used to monitor the transient yields of the CE ion signals to gain a greater understanding of the dynamical timescales involved in CE of the metastable PNT 2+ and PNT 3+ ions. Because the precursor ions cannot be detected directly in TOF-MS, we must rely on the transient signals of the CE product ions to infer the metastable lifetimes of PNT 2+ and PNT 3+ prior to CE.…”

“…As expected, based on the predicted electronic structures of PNT 2+ and PNT 3+ , the CE ion signals of NO 2 + , C 7 H 7 + , and C 7 H 2 2+ are sensitive to the pump–probe delay, although the NO + signal exhibits less change (Supporting Information, Figure S10). The transient ion signals as a function of pump–probe delay τ were fit using the method introduced by Jochim et al where s = 42.4 fs is obtained from the width of the cross-correlation function fwhm = fs. The first term in eq simulates the instrument response function from the cross-correlation signal.…”

“…The time constant T r is associated with the recovery of the ion signal near to its initial value after depletion at short positive delays. This transient depletion and recovery is due to the probe pulse disrupting the spontaneous dissociation of the multiply charged precursor ion. ,− Similarly, transient enhancement and decay with time constant T d are associated with the initial enhancement of an ion signal due to excitation of the multiply charged precursor by the probe pulse, followed by decay due to depletion of the precursor population at longer delays. In this context, it is notable that the NO 2 + signal assigned to the q = 2 precursor has a substantially longer T r = 191 ± 5 fs than NO 2 + assigned to the q = 3 precursor with T r = 89 ± 5 fs.…”

“…Time-resolved “pump–probe” measurements using an intense pump to induce SFI, followed by a weak probe pulse to electronically excite cations, also called “ultrafast disruptive probing”, can identify timescales involved in dissociation pathways of singly- and multiply-charged cations. Using this technique, our group recently found that nitro-nitrite rearrangement in the nitromethane cation with subsequent NO + loss occurs in ∼440 fs and identified coherent vibrational motions in nitrobenzene and nitrotoluene cations with ∼400 fs period, that facilitates direct C–N bond cleavage. − Others have used ultrafast disruptive probing to determine timescales required for H 3 + formation from dications of small alcohols − and pump–probe measurements with extreme-ultraviolet pump pulses to determine CE timescales in methanol, N 2 O, and CO 2 …”

Femtosecond intramolecular rearrangement of the CH3NCS radical cation

“…The dynamics of all the different products following ultrafast ionization were tracked using disruptive probing with the weak 800 nm pulse. 29 The probe pulse was polarized at the "magic" angle (54.7 degrees) relative to the pump pulse to minimize the influence of rotational dynamics on the measurements. The probe was attenuated to about 3×10 13 W cm -2 to ensure that it did not generate ions on its own.…”