Coherent Vibrational and Dissociation Dynamics of Polyatomic Radical Cations

Abstract

The ultrafast dynamics of polyatomic radical cations contribute to important processes including energy transfer in photovoltaics, electron transfer in photocatalysis, radiation‐induced DNA damage, and chemical reactions in the upper atmosphere and space. Probing these dynamics in the gas phase is challenging due to the rapid dissociation of polyatomic radical cations following electron removal, which arises from excess electronic excitation of the molecule during the ionization process. This Concept article introduces the reader to how the pump‐probe technique of femtosecond time‐resolved mass spectrometry (FTRMS) can overcome this challenge to capture coherent vibrational dynamics on the femtosecond timescale in polyatomic radical cations and enable the analysis of their dissociation pathways. Examples of FTRMS applied to three families of polyatomic radical cations are discussed.

Using computational chemistry to design pump–probe schemes for measuring nitrobenzene radical cation dynamics

“…It is common practice in time-resolved studies of radical cations to probe with the readily available 800 nm wavelength. [16][17][18][19][20][21][22][23][24][25][26][27] In this work we want to address whether theoretical computations can predict a more suitable wavelength for probing vibrational wave packet dynamics in radical cations. Indeed, the enhancement of wave packet oscillation amplitudes in NB cation at modest probe pulse intensity (10 12 W cm −2 ) using 650 nm as compared to 800 nm for excitation validates the computational results showing a geometry-dependent excitation probability from the D 0 to D 4 surface and resonance of the 650 nm excitation wavelength.…”

“…To quantify the enhancement of oscillation amplitudes observed in the transient signals in Figure 8 using the 650 nm probe wavelength, nonlinear least squares curve fitting [21][22][23][24][25][26]29 was applied to the signals. The fit equation used for the transient ion signals in Figure 8 is…”

“…[5][6][7][8][9][10][11] The high yield of intact molecular ion is attributed to increased contribution of adiabatic electron tunneling to the ionization process, thereby enhancing pop-ments on radical cations use the readily available 800 nm wavelength for probing cation dynamics. [16][17][18][19][20][21][22][23][24][25][26][27] It would stand to reason that selecting the excitation wavelength to be resonant with electronic transitions in radical cations would enhance the amplitudes of the coherent oscillations resulting from vibrational wave packet dynamics. Computations of electronic potential energy surfaces can efficiently predict geometry-dependent electronic transitions in radical cations, 17,18,21,22 making computational chemistry a potentially powerful tool for designing pumpprobe control schemes to measure coherent vibrational dynamics.…”

Coherent Control of Molecular Dissociation by Selective Excitation of Nuclear Wave Packets

“…Understanding the physical mechanisms underlying coherent control of molecular dissociation can be achieved using two-pulse “pump-probe” excitation schemes Tannor and Rice (1985) , Zewail (1988) . Pump-probe measurements with complementary quantum chemical calculations of the relevant electronic potential energy surfaces (PESs) have revealed bond-cleavage mechanisms facilitated by coherent vibrational motions in numerous organic cations Moore Tibbetts (2019) . For instance, coherent excitation of the I–C–Br bending mode in CH 2 IBr + upon strong-field ionization facilitates dissociation into CH 2 Br + upon excitation of the D 0 →D 3 transition at a specific point on the D 0 PES Nichols et al (2009) .…”

Conformer-Specific Dissociation Dynamics in Dimethyl Methylphosphonate Radical Cation

“…Ultrafast pump-probe spectroscopy [ 6 ] has been an effective experimental technique to study the dynamics occurring on an ultrafast time scale. In particular, strong field ionization followed by dissociation has been a useful technique to probe ultrafast dynamics in radical cations [ 7 , 8 , 9 , 10 , 11 , 12 ]. The pump pulse here ionizes molecules to create radical cations, whose dynamics are then studied with the help of a weak probe pulse that can excite to higher electronic states of the cation to induce dissociation.…”