Session J08: Photon Induced Processes

Dissociation dynamics in the 3-body breakups of methanol upon valence photo double ionization

S. Kumar¹, M. Shaikh¹, W. Iskandar¹, R. Thurston¹, M. A. Fareed¹, D. Call², R. Enoki², C. Bagdia³, N. Iwamoto³, T. Severt³, J. B. Williams², I. Ben-Itzhak³, D. S. Slaughter¹, and Th. Weber<sup>1

 

1</sup>Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA-94720, USA

²Department of Physics, University of Nevada, Reno, NV-89557, USA

³J.R. Macdonald Laboratory, Department of Physics, Kansas State University, Manhattan, NS-66506, USA

We investigated the valence photo double ionization (54eV) of single methanol molecules, which represent the simplest hydrocarbon with a hydroxyl and methyl group, hence can be considered methyl substituted water,  using the COLTRIMS (COLd Target Recoil Ion Momentum Spectroscopy) technique. We were able to detect the momenta of two ejected electrons and nascent photo-fragments in coincidence. We focused on two competing 3-body fragmentation channels resulting in

(a) CH₃OD+ hv (54 eV) -> (CH3OD)*²⁺ + 2e^- -> CH₂⁺ + OD⁺ + H + 2e^-

(b) CH₃OD+ hv (54 eV) ->(CH3OD)*²⁺ + 2e^- -> CH₂⁺ + OD + H⁺ + 2e^-

 

Electron transfer has been reported to contribute to the photodissociation via spin orbit coupling (SOC) [1] , which we recently found to contribute to the photo dissociation of deuterated water molecules. We derived the electronic energy sharing, the electron-ion energy sharing, as well as the fragment pair momentum sharing distributions.

From these observables we concluded that both reactions are initiated on the same manifold of dication states, but that channel (a) is almost solely driven by direct double ionization and results in an instant 3-body breakup. On the other hand, reaction channel (b) shows a contribution from both, direct double ionization and autoionization, which mainly initiates a sequential breakup into  with a subsequent electron transfer from the hydroxyl group to the proton.

From Momentum sharing maps it is clearly visible that for reaction (a) H/OD+ are highly correlated and are dominant in comparison to reaction (b)

Investigations of the relative fragment emission angles and recoil frame photoelectron angular distributions as well as a native frame analysis are underway to further track down the role of SOC in the charge transfer process.

References:

[1] W. Iskander et.al. J. Chem. Phys. 159, 094301 (2023).   

Asymmetry parameter (β2) as a tracer of bond dissociation for CH3F

We investigate the effect of the dissociation of the C-F bond on the photoionization asymmetry parameter (β₂) of CH₃F. We aim to determine whether such a parameter can be used as a fingerprint of bond breaking. We engage the cc-pVTZ basis [1] for the construction of molecular orbitals (MOs) for CH₃F⁺ for a set of C-F bond lengths (R_C−F ). A set of extra target orbitals (3 for a′, 1 for a′′ ) has been incorporated to model the static-exchange and polarization potential for the e-CH₃F⁺ system. We make use of a hybrid (GTO+BTO) orbital approach [2] for generating the continuum orbital representing the photoelectron. The calculations are carried out using the UKRmol+ scattering suite [3]. We observe change in β₂ with the C-F bond length as well as signs of dissociation along the C-F bond in the asymmetry parameter.    

Electron vortices in Atomic Ionization by a Single Few-Cycle Circularly Polarized Laser Pulse

Quantum vortices may occur not only in the wave functions but also in the amplitudes of various fragmentation processes, such as impact ionization$^{1-3}$. The structure of quantum vortices in the amplitude of the photodetachment by CP pulses was analyzed in the series of works$^{4-5}$, where the process was considered both, in multiphoton, and strong-field regime. Formation of electron vortices in the strong-field ionization was the subject of recent works$^{6-7}$. Here, the emergence of electron vortices in the ionization amplitude for an atom by an isolated few-cycle circularly polarized (CP) laser pulse is analyzed in the multiphoton regime within the perturbation theory (PT) framework. We demonstrate that the number of quantum vortices, vortex position and topological charge are determined by the relative magnitudes of the dynamical amplitude parameters corresponding to sequential photon absorption. By looking at the momentum distribution of the ionization amplitude phase, we show that these distributions exhibit spiral structures in the form of “Coulomb spirals”, which are signatures of the Coulomb scattering phases. We find that in region where the interference of dynamical variables of the ionization amplitude does not occur, the number of spiral arms correspond to the number of absorbed photons. In the region where the interference does occurs, the formation of an additional spiral arm is reported, which is due to quantum vortices being formed.

[1] J.M. Feagin, J. Phys. B \textbf{44}, 011001 (2010)

[2] J. H. Macek et al., Phys. Rev. Lett. \textbf{104}, 033201 (2010)

[3] J. S. Briggs and J. M. Feagin, Phys. Rev. A \textbf{90}, 052712 8 (2014).

4] L. Geng, F. Cajiao Velez, J. Z. Kaminski, L.-Y. Peng, and K. Krajewska, Phys. Rev. A \textbf{104}, 033111 (2021).

[5] L. Geng, F. Cajiao Velez, J. Z. Kaminski, L.-Y. Peng, and K. Krajewska, Phys. Rev. A \textbf{102}, 52 043117 (2020).

[6] O. I. Tolstikhin and T. Morishita, Phys. Rev. A 99, 063415 (2019). 

[7] Y. Kang, E. Pisanty, M. Ciappina, M. Lewenstein, C. F. de Morisson Faria, and A. S. Maxwell, Eur. Phys. \textbf{59} J. D 75, 199 (2021).   

Efficient Autoionization and Ion Trap Loading of 133Ba+

The ¹³³Ba⁺ trapped ion qubit provides many advantageous qualities for trapped ion quantum technology, including  nuclear spin I=1/2, visible wavelength cooling transitions, and a long lived (τ =30 s) metastable D5/2 state. This also makes it the ideal candidate to implement the omg (optical-metastable-ground) architecture that utilizes three qubits in a single atom. However due to its low abundance, scaling technology utilizing this ion requires highly efficient photoionization. We present advances in the photoionization and loading of barium trapped ions. We will highlight a novel scheme for two step photoionization through the 5d6p³D₁ level to multiple autoionizing states. This resonant photoionization technique allows for isotope selective ionization of low abundance barium isotopes.    

Single photon double ionization of lithium hydride molecule

Building on work for double ionization of molecular lithium, Li₂, we report the triple differential cross sections (TDCS) for double photoionization of the valence electrons of lithium hydride.  The LiH molecule possesses a more traditional chemical bond than the weakly bound Li₂ molecule and thus has more in common for the initial state environment with second-row diatomic molecules, including its equilibrium internuclear distance and double ionization potential.  Additionally, LiH offers the possibility to study double ionization from a molecule without parity symmetry and consider the angular distributions towards or away from the heavier Li side.  Insight can also be gained using a decomposition of the initial state correlation using natural orbitals to meaningfully consider different levels of electron correlation and their impact on the resulting angular cross sections computed in the body frame.   

Asymmetric Photoelectron Momentum Distribution from Photoionization of Carbon Monoxide

Asymmetry in the photoelectron momentum distribution (PMD) is a remarkable feature of strong-field and attosecond science especially when produced from heteronuclear diatomic molecules due to their electric dipole moments. It has been studied from the nonlinear process of multiphoton single-ionization of the CO molecule$^{1}$ by exploring its laser-polarization dependence, to the linear process of single-photon single-ionization (dubbed photoionization) of the CO molecule$^{2,3}$ by examining its Stereo-Wigner time delay, and the nitric oxide (NO) molecule$^{4}$ by looking at the shape resonance and photoionization time delay. While NO is an open-shell molecule, CO is a closed-shell one, and the rather small energy gap between its highest occupied molecular orbital (HOMO) and the orbital below it (HOMO-1) compared to the broader UV pulse bandwidth renders possible single ionization by one-photon transition from these inner and outer orbitals. Using the \emph{ab initio} R-matrix method with time dependence (RMT)$^{5}$, we have examined photoionization of CO by a single UV laser pulse for several orientations of the linear polarization vector with respect to the molecular axis. The PMD is found to be entangled (mixed) because electrons are emitted from these inner and outer initial orbitals. We show that the asymmetry of the HOMO is strongly revealed in the entangled PMD for any molecular orientation. Such basic feature characteristic of HOMO can be used as a marker to determine the degree-of-mixture between the ionization signals due to HOMO and HOMO-1$^6$.

[1] Zhu et al, Optica \textbf{19}, 24198 (2011)

[2] Chacon and Ruiz, Optics Express \textbf{26}, 4548 (2018)

[3] Vos et al., Science \textbf{360}, 1326 (2018)

[4] Gong et al., Phys. Rev. X \textbf{12}, 011002 (2022)

[5] A. C. Brown \textit {et al.}, Comp. Phys. Comm., {\bf 250}, 107062 (2022)

[6] H. B. Ambalampitiya and J. M. N. Djiokap, (manuscript submitted)   

Ionization energy measurement of optically pumped rubidium molecules in a beam

In this work, we have measured the ionization energy of Rb₂ molecules optically pumped in a supersonic beam. First, we use optically shaped broadband multimode diode lasers to rovibrationally cool the molecules trough transitions between the X¹Σ_g⁺ and the B¹Π_u  potentials. By optically shaping the multimode diode lasers, the X¹Σ_g⁺(v_X=0,J_X<10) states are dark, and the molecules are pumped into them. Then a pulsed dye laser excites the X¹Σ_g⁺(v_X=0,J_X<10) →B¹Π_u (v_B=4) transition, which is used because it allows better isotope discrimination (⁸⁵Rb₂). A second dye laser, synchronized with the first, excites the B¹Π_u (v_B=4) → X²Σ_g⁺(v_X=0) transition, photoionizing the molecule. The ionization happens at zero electric field, to avoid the excitation Rydberg molecular states. About 70 ns after the ionization, we apply a fast high voltage pulse to detect the molecular ions using a channeltron and a counter. The second laser is scanned in the 591 and 609 nm range. The results suggests that ionization energy is higher (∽60 cm^-1) than previous results in the literature, and they will be discussed during the presentation.   

Ultra fast variable repetition rate Ytterbium doped fiber laser

We present the design, implementation, and performance of a novel high-power ultrafast Ytterbium (Yb3+) doped fiber laser. This system uses a polarization-maintained Ytterbium doped fiber and an AOM to operate at 90W average power at 1030 nm with variable repetition rates from 20 kHz-50 MHz and pulse energy of ~1mJ in 300 femto-second pulses.   

Non-Markovian open quantum dynamics in squeezed environments: coherent state unraveling

We apply the stochastic Schr ̈odinger equation approach to study the non-Markovian dynamics of quantum systems coupled to a squeezed environment. We derive the non-Markovian quantum-state-diffusion equation using coherent-state-unraveling and the associated zeroth order master equation for general models in a microscopic quantum context. Focused on a dissipative optical cavity coupled with a squeezed vacuum, the numerical simulations demonstrate the time evolution of photon numbers in an optical cavity. We observe that the long-time limits of the non-Markovian dynamics are significantly different from those of the Markovian dynamics for various memory factors and squeezing factors. Additionally, non-Markovian dynamics exhibit a distinct pattern of behavior when parameters change. Our work provides a method to study the impact of multiple parameters on the non-Markovian dynamics and long-time limits jointly. The method can be further extended to finite-temperature environments.