Quantum state-to-state vacuum ultraviolet photodissociation dynamics of small molecules

Exploring the vacuum ultraviolet photochemistry of astrochemically important triatomic molecules

The recently constructed vacuum ultraviolet (VUV) free electron laser (FEL) at the Dalian Coherent Light Source (DCLS) is yielding a wealth of new and exquisitely detailed information about the photofragmentation dynamics of many small gas-phase molecules. This Review focuses particular attention on five triatomic molecules-H2O, H2S, CO2, OCS and CS2. Each shows excitation wavelength-dependent dissociation dynamics, yielding photofragments that populate a range of electronic and (in the case of diatomic fragments) vibrational and rotational quantum states, which can be characterized by different translational spectroscopy methods. The photodissociation of an isolated molecule from a well-defined initial quantum state provides a lens through which one can investigate how and why chemical reactions occur, and provides numerous opportunities for fruitful, synergistic collaborations with high-level ab initio quantum chemists. The chosen molecules, their photofragments and the subsequent chemical reaction networks to which they can contribute are all crucial in planetary atmospheres and in interstellar and circumstellar environments. The aims of this Review are 3-fold: to highlight new photochemical insights enabled by the VUV-FEL at the DCLS, notably the recently recognized central atom elimination process that is shown to contribute in all of these triatomic molecules; to highlight some of the potential implications of this rich photochemistry to our understanding of interstellar chemistry and molecular evolution within the universe; and to highlight other and future research directions in areas related to chemical reaction dynamics and astrochemistry that will be enabled by increased access to VUV-FEL sources.

A New Band System of the Dicarbon Molecule in the Vacuum Ultraviolet Region

As one of the most abundant molecules in the universe, the long history of spectroscopic studies of the dicarbon molecule, C₂, reaches back two centuries. While many electronic band systems with upper states below the lowest dissociation threshold have been well characterized, much less is known about transitions to higher-lying states. Here, we report the observation of a new band system of C₂ from the lowest triplet state a³Π_u through a resonance-enhanced multiphoton ionization scheme. The upper state is identified as 1³Σ_g⁺, which is determined to be 61539.0 cm^-1 (7.630 eV) above ground state X¹Σ_g⁺. The spectroscopic parameters determined for the 1³Σ_g⁺ state are in excellent agreement with those predicted by the high-level ab initio calculations. This study paves the way for systematic investigations of the photoabsorption and photodissociation of C₂ in the vacuum ultraviolet region, which has important applications in the field of astrochemistry.

Vacuum ultraviolet photoexcitation and photofragment spectroscopic studies of 14N15N between 109000 and 117500 cm-1

In this study, we employed a newly built time-slice velocity-map ion imaging setup, equipped with two tunable vacuum ultraviolet (VUV) laser sources, to obtain the first comprehensive high-resolution photoexcitation and photofragment excitation spectra of ¹⁴N¹⁵N in the VUV photon energy range 109 000-117 500 cm^-1. The spectroscopic simulation program PGOPHER was used to analyze the rotationally resolved spectra. Band origins, rotational constants, and isotope shifts compared with those of ¹⁴N₂ have been obtained for 31 electric-dipole-allowed vibrational states of ¹⁴N¹⁵N in the aforementioned energy range. These spectroscopic parameters are found to depend on the vibrational quantum number irregularly. Systematic perturbations of the rotational transition energies and predissociation rates within individual absorption bands have also been observed. These are proved to be caused by the strong homogeneous interactions between the valence b'¹Σ_u ⁺ state and the Rydberg c_n ^{' 1}Σ_u ⁺ states, and between the valence b¹Π_u states and the Rydberg o₃ ¹Π_u states. Heterogeneous interactions between the Rydberg c_n ¹Π_u states and c_n ^{' 1}Σ_u ⁺ states also play an important role.

Vacuum Ultraviolet Photodissociation Branching Ratios of 12C16O, 13C16O, and 12C18O from 100500 to 102320 cm-1

The C⁺ ion photofragment spectra and photodissociation branching ratios into the two energetically available channels, C(¹D) + O(³P) and C(³P) + O(³P), have been obtained for the three CO isotopologues, ¹²C¹⁶O, ¹³C¹⁶O, and ¹²C¹⁸O, in the vacuum ultraviolet range 100500-102320 cm^-1. The two vibronic states of ¹Σ⁺ symmetry, F(3dσ) ¹Σ⁺(υ' = 1) and J(4sσ) ¹Σ⁺(υ' = 0), predominantly dissociate into the lowest channel C(³P) + O(³P) through interactions with the repulsive D'¹Σ⁺ state. All three vibronic states of ¹Π symmetry, E'¹Π(υ' = 1, 2) and G(3dπ) ¹Π(υ' = 0), dissociate into both of the channels above. The photodissociation branching ratios into the channel C(¹D) + O(³P) for E'¹Π(υ' = 1, 2) are found to be independent of both the rotational quantum number and e/f parity, while those for G(3dπ) ¹Π(υ' = 0) strongly depend on the rotational quantum number, indicating very different predissociation pathways between the valence states E'¹Π(υ' = 1, 2) and the Rydberg state G(3dπ) ¹Π(υ' = 0). The potential energy curves of CO in the aforementioned energy range and below have recently been well constructed due to a series of interplays between high-resolution spectroscopic studies and theoretical calculations; the photodissociation branching ratios measured in this study can provide further benchmarks for future theoretical investigations which aim to understand the detailed predissociation dynamics of CO.

Reinvestigation of the Rydberg W1Π(ν = 1) level of 12C16O, 13C16O, and 12C18O through rotationally dependent photodissociation branching ratio measurements

A recent high resolution photoabsorption study revealed that the Rydberg W¹Π(ν = 1) level of carbon monoxide (CO) is perturbed by the valence E″¹Π(ν = 0) level, and the predissociation linewidth shows drastic variation at the crossing point due to the interference effect [Heays et al., J. Chem. Phys. 141(14), 144311 (2014)]. Here, we reinvestigate the Rydberg W¹Π(ν = 1) level for the three CO isotopologues, ¹²C¹⁶O, ¹³C¹⁶O, and ¹²C¹⁸O, by measuring the rotationally dependent photodissociation branching ratios. The C⁺ ion photofragment spectra obtained here reproduce the recent high resolution photoabsorption spectra very well, including the presence of the valence E″¹Π(ν = 0) level. The photodissociation branching ratios into the spin-forbidden channel C(¹D) + O(³P) show sudden increases at the crossing point between the W¹Π(ν = 1) and E″¹Π(ν = 0) levels, which is in perfect accordance with the drastic variation of the linewidth observed in the recent spectroscopic study. Further analysis reveals that the partial predissociation rate into the lowest channel C(³P) + O(³P) shows a much more prominent decrease at the crossing point, which is caused by the interference effect between the W¹Π(ν = 1) and E″¹Π(ν = 0) levels, than that into the spin-forbidden channel C(¹D) + O(³P), and this is the reason of the sudden increase as observed in the photodissociation branching ratio measurements. We hope that the current experimental investigation will stimulate further theoretical studies, which could thoroughly address all the experimental observations in a quantitative way.

Channel-resolved rotationally dependent predissociation rate constants reveal the state-to-state dissociation dynamics of carbon monoxide in electronically excited states

Rotational dependence of the total predissociation rate constants deduced from linewidth measurements in spectroscopic studies have often been used to predict the possible electronic coupling schemes in the photodissociation process of carbon monoxide (CO), while the intrinsic multi-channel characteristics of CO photodissociation make the prediction unreliable and sometimes misleading conclusions could be reached. Here, we demonstrate for the first time in the Rydberg 4p(2) and 5p(0) complexes region of ¹³C¹⁶O that absolute partial predissociation rate constants into each individual channel and their dependences on the rotational quantum numbers can be obtained through branching ratio measurements in combination with the total predissociation rate constants reported in spectroscopic studies. These channel-resolved rotationally dependent predissociation rate constants are found to unambiguously reveal the detailed state-to-state photodissociation dynamics of CO, which is not available from either the branching ratio or the total predissociation rate constant measurements individually.

Photodissociation of HCl in the photon energy range 14.6-15.0 eV: Channel-resolved branching ratios and fragment angular distributions

For the HCl molecule, four photodissociation channels are open in the excitation energy region 14.6-15.0 eV: H(2s) + Cl(²P_3/2), H(2p) + Cl(²P_3/2), H(2s) + Cl(²P_1/2), and H(2p) + Cl(²P_1/2). We measured the fragment angular distributions and the branching ratios of the four dissociation channels by using the extreme ultraviolet laser pump and UV laser probe, delay-time-curve, and velocity map imaging methods. The channel-resolved fragment angular distributions and fragment yield spectra show that various Rydberg states (superexcited states) contribute to the absorption cross sections, including the [A²Σ⁺]4pσ, [A²Σ⁺]4pπ, [A²Σ⁺]3dσ, [A²Σ⁺]3dπ, and [A²Σ⁺]5sσ states. Most of the H(2s) + Cl(²P_1/2) channels correlate with the ¹Σ⁺ states, while the other channels correlate with mixing excitations of the ¹Σ⁺ and ^1,3Π states. The channel branching ratios are dependent on the excitation energies. When the four channels are open, the channel branching ratios of H(2s) + Cl(²P_3/2) and H(2p) + Cl(²P_1/2) are small. Based on the recent ab initio potential energy curves, the Rydberg states converging to the ion-core A²Σ⁺ are proposed to be predissociated by the nuclear vibrational continua of the Rydberg states converging to the ion-core X²Π.

Strong and selective isotope effect in the vacuum ultraviolet photodissociation branching ratios of carbon monoxide

Rare isotope (¹³C, ¹⁷O and ¹⁸O) substitutions can substantially change absorption line positions, oscillator strengths and photodissociation rates of carbon monoxide (CO) in the vacuum ultraviolet (VUV) region, which has been well accounted for in recent photochemical models for understanding the large isotopic fractionation effects that are apparent in carbon and oxygen in the solar system and molecular clouds. Here, we demonstrate a strong isotope effect associated with the VUV photodissociation of CO by measuring the branching ratios of ¹²C¹⁶O and ¹³C¹⁶O in the Rydberg 4p(2), 5p(0) and 5s(0) complex region. The measurements show that the quantum yields of electronically excited C atoms in the photodissociation of ¹³C¹⁶O are dramatically different from those of ¹²C¹⁶O, revealing strong isotope effect. This isotope effect strongly depends on specific quantum states of CO being excited, which implies that such effect must be considered in the photochemical models on a state by state basis.