Daniel M. Neumark’s research while affiliated with Lawrence Berkeley National Laboratory and other places

Molecular beam scattering experiments are carried out to study collisions between Ne atoms (Ei = 24.3 kJ mol⁻¹) and the surface of a cold salty water (8 m LiBr(aq), 230 K) flat jet. Translational energy distributions are collected as a function of scattering angle using a rotatable mass spectrometer. Impulsive scattering and thermal desorption contribute to the overall scattering distributions, but impulsive scattering dominates at all three incidence angles explored. Highly super-specular scattering is observed in the impulsive scattering channel that is attributed to anisotropic momentum transfer to the liquid surface. The thermal desorption channel exhibits a cos θ angular distribution. Compared to Ne scattering from dodecane, fractional energy loss in the impulsive scattering channel is much larger across a wide range of deflection angles. A soft-sphere model is applied to investigate the kinematics of energy transfer between the scatterer and liquid surface. Fitting to this model yields an effective surface mass of 250−60⁺¹⁰⁰ amu and internal excitation of 11.8 ± 1.6 kJ mol⁻¹, both of which are considerably larger than for Ne/dodecane. It thus appears that energy transfer to cold salty water is more efficient than to a dodecane liquid surface, a result attributed to the extensive hydrogen-bonded network of liquid water and roughness of the liquid surface.

Accessing coherences is key to fully understand and control ultrafast dynamics of complex quantum systems like molecules. Most photochemical processes are mediated by conical intersections (CIs), which generate coherences between electronic states in molecules. We show with accurate calculations performed on gas-phase methyl iodide that CI-induced electronic coherences of spin-orbit-split states persist in atomic iodine after dissociation. Our simulation predicts a maximum magnitude of vibronic coherence in the molecular regime of 0.75% of the initially photoexcited state population. Upon dissociation, one third of this coherence magnitude is transferred to a long-lived atomic coherence where vibrational decoherence can no longer occur. To trace these dynamics, we propose a table-top experimental approach--heterodyned attosecond four-wave-mixing spectroscopy (Hd-FWM). This technique can temporally resolve small electronic coherence magnitudes and reconstruct the full complex coherence function via phase cycling. Hence, Hd-FWM leads the way to a complete understanding and optimal control of spin-orbit-coupled electronic states in photochemistry.

A setup for extreme-ultraviolet time-resolved photoelectron spectroscopy (XUV-TRPES) of liquids is described based on a gas-dynamic flat jet formed by a microfluidic chip device. In comparison to a cylindrical jet that has a typical diameter of 10-30 micrometers, the larger surface area of the flat jet with a width of ca. 300 micrometers allows for full overlap of the target with the pump and probe light beams. This results in an enhancement of photoelectrons emitted from the liquid, while simultaneously allowing smaller sample consumption compared with other flat jet techniques utilizing liquid collisions or converging slits. Femtosecond pulses of XUV light at a photon energy of 21.7 eV are prepared by high harmonic generation and a multilayer mirror that selects a single harmonic; the He gas used to form the gas-dynamic flat jet is transparent at this energy. Compared to a cylindrical jet, the photoelectron signal from the liquid is enhanced relative to that from the surrounding vapor jacket. Pump-probe spectra for aqueous thymine show notably higher signals for the flat vs cylindrical jet. Moreover, the time-dependent space-charge shift in UV pump/XUV probe experiments is smaller for the gas dynamic flat jet than for a cylindrical jet with the same flow rate, an effect that is accentuated at higher He backing pressures that yield a thinner jet. This reflects reduced multiphoton ionization of the solute by the UV pump pulse, the primary cause of the space charge shift, as the jet becomes thinner and reaches the thickness of a few tens of nm.

In order to explore the vibrational levels of vinylidene (H2CC) and their possible couplings with acetylene (HCCH), we investigate the effect of infrared (IR) vibrational pre-excitation on the high-resolution photoelectron spectra of the vinylidene anion (H2CC¯) and its deuterated isotopologue (D2CC¯). The photoelectron spectra are obtained using slow electron velocity-map imaging of cryogenically cooled anions (cryo-SEVI); here, cold anions are vibrationally excited by an IR laser pulse prior to photodetachment (IR cryo-SEVI). Infrared action spectra of the anion CH2 stretching fundamentals are measured by monitoring growth and depletion of features in photoelectron spectra as the IR laser is tuned, yielding excitation frequencies of the symmetric (ν1) and antisymmetric (ν5) CH2 stretching modes of 2590±2 cm-1 and 2658±2 cm-1, respectively. We then use IR cryo-SEVI to explore the effect of vibrational excitation of the two modes on the anion photoelectron spectrum. Interpretation of these spectra is facilitated by quantum calculations performed for each isotopologue on accurate six-dimensional potential energy surfaces of both neutral and anionic vinylidene. IR cryo-SEVI spectra resulting from excitation of these two close-lying anion vibrations are noticeably different. Excitation of the ν1 mode leads to several new features that appear in the photoelectron spectra which closely match the Franck-Condon allowed transitions predicted by theory. Excitation of the ν5 mode in H2CC¯ reveals complicated spectral features in the vicinity of the 5_1^1 sequence band that are not seen for D2CC¯. These are explained by a combination of anharmonic coupling between ν5 and ν6 (CH2 rock) states in neutral H2CC and possible coupling to the HCCH isomer.

Molecular beam scattering experiments are carried out to study collisions between Ne atoms and the surface of a cold salty water flat jet. Translational energy distributions are collected as a function of scattering angle using a rotatable mass spectrometer. Impulsive scattering and thermal desorption contribute to the overall scattering distributions, but impulsive scattering dominates at all three incidence angles explored. Highly super-specular scattering is observed in the impulsive scattering channel that is attributed to anisotropic momentum transfer to the liquid surface. The thermal desorption channel exhibits a cosθ angular distribution. Compared to Ne scattering from dodecane, fractional energy loss in the impulsive scattering channel is much larger across a wide range of deflection angles. A soft-sphere model is applied to investigate the kinematics of energy transfer between the scatterer and liquid surface. Fitting to this model yields an effective surface mass of 223 +100/−60 amu and internal excitation of 11.6 ± 1.6 kJ mol−1, both of which are considerably larger than for Ne/dodecane. It thus appears that energy transfer to cold salty water is more efficient than to a dodecane liquid surface, a result attributed to the extensive hydrogen-bonded network of liquid water and roughness of the liquid surface.

In order to explore the vibrational levels of vinylidene (H2CC) and their possible couplings with acetylene (HCCH), we investigate the effect of infrared (IR) vibrational pre-excitation on the high-resolution photoelectron spectra of the vinylidene anion (H2CC¯) and its deuterated isotopologue (D2CC¯). The photoelectron spectra are obtained using slow electron velocity-map imaging of cryogenically cooled anions (cryo-SEVI); here, cold anions are vibrationally excited by an IR laser pulse prior to photodetachment (IR cryo-SEVI). Infrared action spectra of the anion CH2 stretching fundamentals are measured by monitoring growth and depletion of features in photoelectron spectra as the IR laser is tuned, yielding excitation frequencies of the symmetric (ν1) and antisymmetric (ν5) CH2 stretching modes of 2590±2 cm-1 and 2658±2 cm-1, respectively. We then use IR cryo-SEVI to explore the effect of vibrational excitation of the two modes on the anion photoelectron spectrum. Interpretation of these spectra is facilitated by quantum calculations performed for each isotopologue on accurate six-dimensional potential energy surfaces of both neutral and anionic vinylidene. IR cryo-SEVI spectra resulting from excitation of these two close-lying anion vibrations are noticeably different. Excitation of the ν1 mode leads to several new features that appear in the photoelectron spectra which closely match the Franck-Condon allowed transitions predicted by theory. Excitation of the ν5 mode in H2CC¯ reveals complicated spectral features in the vicinity of the 5_1^1 sequence band that are not seen for D2CC¯. These are explained by a combination of anharmonic coupling between ν5 and ν6 (CH2 rock) states in neutral H2CC and possible coupling to the HCCH isomer.

The nitrate (NO3) radical has long been the subject of both experimental and theoretical studies due to its complex electronic structure resulting from vibronic interactions between the 𝑋 ̃ 2𝐴′2 and 𝐵 ̃ 2𝐸′ states. In particular, the definite assignment of the fundamental of its degenerate stretching vibration (𝜈3) is still under debate. Here, we extend the available spectroscopic information by reporting high-resolution photoelectron spectra of vibrationally pre-excited NO3− using the recently developed IR-cryo-SEVI technique. The anions are excited through infrared (IR) excitation near 1350 cm−1, accessing the 𝜈3 and 2𝜈3(𝑒′) vibrational levels with band centers at 1350.5 cm−1 and ~2700 cm−1, respectively. The IR-cryo-SEVI spectrum for 2𝜈3 pre-excitation shows clear evidence for an intense 3_2^1 transition. From the position of this feature (30031 cm−1), the electron affinity of NO3 also determined in this work (31680 cm−1), and the IR excitation energy, we obtain a fundamental frequency of 1051 cm−1 for the 𝜈3 fundamental of the NO3 radical. This assignment and other features in the IR-cryo-SEVI spectra are supported by spectral simulations based on a vibronic Köppel-Domcke-Cederbaum Hamiltonian. The simulations also show that nearly all features in the IR-cryo-SEVI spectra arise because of pseudo-Jahn-Teller coupling between the 𝑋 ̃ and 𝐵 ̃ state of NO3. The results and analysis presented here settle a long-standing controversy regarding the 𝜈3 frequency of NO3.

The helium atom, with one nucleus and two electrons, is a prototypical system to study quantum many-body dynamics. Doubly excited states, or quantum states in which both electrons are excited by one photon, showcase electronic-correlation mediated effects. In this paper, the natural lifetimes of the doubly excited 1 P o 2 s n p Rydberg series and the 1 S e 2 p 2 dark state in helium in the 60–65 eV region are measured directly in the time domain with extreme-ultraviolet/near-infrared noncollinear attosecond four-wave-mixing (FWM) spectroscopy. The measured lifetimes agree with lifetimes deduced from spectral linewidths and theoretical predictions, and the roles of specific decay mechanisms are considered. While complex spectral line shapes in the form of Fano resonances are common in absorption spectroscopy of autoionizing states, the background-free and thus homodyned character of noncollinear FWM results exclusively in Lorentzian spectral features in the absence of strong-field effects. The onset of strong-field effects that would affect the extraction of accurate natural lifetimes in helium by FWM is determined to be approximately 0.3 Rabi cycles. This study provides a systematic understanding of the FWM parameters necessary to enable accurate lifetime extractions, which can be utilized in more complex quantum systems in the future. Published by the American Physical Society 2024