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The journal of physical chemistry / A 124(40), 8341-8345 (2020). doi:10.1021/acs.jpca.0c05893 special issue: "Daniel Neumark Festschrift", Methyl vinyl ketone (C$_4$H$_6$O) is a volatile, labile organic compound of importance in atmospheric chemistry. We prepared a molecular beam of methyl vinyl ketone with a rotational temperature of 1.2(2) K and demonstrated the spatial separation of the s-cis and s-trans conformers of methyl vinyl ketone using the electrostatic deflector. The resulting sample density was 1.5(2) × 10$^8$ cm$^{−3}$ for the direct beam in the laser ionization region. These conformer-selected methyl vinyl ketone samples are well suited for conformer-specific chemical reactivity studies such as in Diels−Alder cycloaddition reactions., Published by Soc., Washington, DC

Physical chemistry, chemical physics 24, 23096 - 23105 (2022). doi:10.1039/D2CP02252D special issue: "Ions, electrons, coincidences and dynamics: Festschrift for John H.D. Eland", We investigated the dissociation of dications and trications of three polycyclic aromatic hydrocarbons (PAHs), fluorene, phenanthrene, and pyrene. PAHs are a family of molecules ubiquitous in space and involved in much of the chemistry of the interstellar medium. Ions are formed by ionization with 30.3 nm extreme ultraviolet (XUV) photons, and their velocity map images are recorded using a PImMS multi-mass imaging sensor. Application of recoil-frame covariance analysis allows the total kinetic energy release (TKER) associated with multiple fragmentation channels to be determined to high precision. Experimental measurements are supported by Born-Oppenheimer molecular dynamics (BOMD) molecular dynamics simulations., Published by RSC Publ., Cambridge

Journal of physics / B 55(14), 144001 - (2021). doi:10.1088/1361-6455/ac6b6b, We demonstrate the application of event-driven Timepix3-based detectors in combination with a double-sided velocity-map-imaging spectrometer to record the full 3D momentum of charged particles at the free-electron-laser facility FLASH. We measured the XUV induced fragmentation of $\text{N}_2$ using 250 kHz FLASH bursts with sub-pixel spatial resolution and up to 1.7 ns temporal resolution for photoelectrons. To further demonstrate the capabilities of this camera at even higher repetition rates we measured single-shot images of He($1s$) photoelectrons for bursts with a repetition rate of 1 MHz. Overall, with the Timepix3 camera we overcome limitations of standard-camera technology for advanced-imaging experiments with requirements on high event-rates and high spatio-temporal resolution., Published by IOP Publ., Bristol

Strong field double ionization that triggers the chemical bond rearrangement of CH3Cl is investigated by impulsive control of the alignment of molecules. The alignment and laser intensity dependent H2+ and H3+ yields in linearly polarized femtosecond laser have been measured, and the obtained data show that the maximum signal of H2+ appears at the laser polarization parallel to the C–Cl axis of molecules and H3+ species are more likely to eject at the laser polarization parallel to the C–Cl axis at low laser intensity while the H3+ signal peaks at laser polarization perpendicular to the C–Cl axis at high laser intensity. The measurements indicate that electrons from HOMO – 1 and HOMO – 2 orbitals have been ionized for the generation of bond rearrangement at different laser intensity. Our results demonstrate the importance of multielectron effects and also provide an effective control method in the process of chemical bond rearrangement of the molecules in strong laser fields.

Strong field sequential ionization of symmetric-top CH3I molecules is studied experimentally by using a combined method of femtosecond laser-induced impulsive alignment and time-of-flight mass spectrometry. Both alignment- and angular-dependent ion yields have been measured, and the sequential ionization of a multielectron has been discussed. It is found that the maximum ionization occurs when the polarization of probe laser is perpendicular to the internuclear axis of molecules, and the signal of fragment ion peaks at the polarization of the probe laser is parallel to the internuclear axis of molecules. The angular distribution of ions indicated that ionization of π-type orbitals corresponds to generation of charged parent ions and ionization of σ-type orbitals corresponds to generation of fragment ions. The sequential release of multielectrons for Coulomb explosion channels is studied by analysis of the time evolutions of multicharged In+ (n = 1–4) signals.

Journal of molecular structure 1208, 127863 (2020). doi:10.1016/j.molstruc.2020.127863 special issue: "VSI: Jon Hougen.", The spatial separation of 2-propanol monomer from its clusters in a molecular beam by an electrostatic deflector was demonstrated. Samples of 2-propanol monomer with a purity of 90% and a beam density of $7 \times 10^6$ cm$^{-3}$ were obtained. These samples were utilized to study the femtosecond-laser-induced strong-field multi-photon ionization and fragmentation of 2-propanol using non-resonant 800 nm light with peak intensities of $3–7 \times 10^{13}$ W/cm$^2$., Published by Elsevier, New York, NY [u.a.]

We study the multiphoton ionization of potassium atoms in 800 nm and 400 nm femtosecond laser fields. In the 800 nm laser field, the potassium atom absorbs three photons and emits one electron via one photon resonance with the 4p intermediate state with the help of the ac-Stark shift. The resonance feature is clearly shown as an Autler–Townes (AT) splitting and is mapped out in the electron kinetic energy spectrum. In a 400 nm laser field, although one photon resonance is possible with the 5p state, no splitting is observed. The different transition amplitudes between 4s–4p and 4s–5p explain the observed results. Due to the AT effect, an unexpected peak in the photoelectron energy spectrum that violates the dipole transition rule is observed. A preliminary explanation involving the spin-orbit interaction in the p state is given to account for this component. The observed AT-splitting in the electron kinetic energy distribution can be used as an effective method to calibrate the intensity of a laser field.

Rotational dynamics of quantum state selected and unselected CH3I molecules in intense femtosecond laser fields has been studied. The orientation and alignment evolutions are derived from a pump-probe measurement and in good agreement with the numerical results from the time-dependent Schrodinger equation (TDSE) calculation. The different rotational transitions through nonresonant Raman process have been assigned from the Fourier analysis of the orientation and alignment revivals. These revivals are derived from a pump-probe measurement and in good agreement with the numerical results from the TDSE calculation. For the molecules in rotational state vertical bar 1, +/- 1, -/+ 1 >, the transitions can be assigned to Delta J = +/- 1, +/- 2, while for thermally populated molecules, the transitions are Delta J = +/- 2. Our results illustrate that the orientation and alignment revivals of the rotational quantum-state selected molecules give a deep insight into the rotational excitation pathways for the transition of different rotational states of molecules in ultrafast laser fields.

We have measured the alignment influence on ion yields of CH3I and CH3Br molecules in the laser intensity regime from 1013 W/cm2 to 1015 W/cm2. The hexapole state-selection technique combined with laser induced alignment has been employed to obtain aligned (〈P2(cosθ)〉=0.7) and anti-aligned (〈P2(cosθ)〉=-0.1) CH3I and CH3Br molecules. The ratio of saturation intensities observed for the CH3I and CH3Br molecules with different alignments are simulated using a modified PPT model for molecules considering different orbitals and a good agreement has been achieved, indicating the contribution from multiple orbitals in the ionization.

Spatial separation of water dimer from water monomer and larger water-clusters through the electric deflector is presented. A beam of water dimer with $93~\%$ purity and a rotational temperature of $1.5~$K was obtained. Following strong-field ionization using a $35~$fs laser pulse with a wavelength centered around $800~$nm and a peak intensity of $10^{14}~\text{W}/\text{cm}^2$ we observed proton transfer and $46~\%$ of the ionized water dimer broke apart into a hydronium ion $\text{H}_3\text{O}^+$ and neutral OH.

Strong field double ionization that triggers the chemical bond rearrangement of CH

Hexapole state selection is used to prepare CH3I molecules in the |JKM〉 = |1±1∓1〉 state. The molecules are aligned in a strong 800 nm laser field, which is linearly polarised perpendicular to the weak static extraction field E of the time of flight setup. The molecules are subsequently ionised by a second time delayed probe laser pulse. It will be shown that in this geometry at high enough laser intensities the Newton sphere has sufficient symmetry to apply the inverse Abel transformation to reconstruct the three dimensional distribution from the projected ion image. The laser induced controllable alignment was found to have the upper and lower extreme values of 〈P2(cos )〉 = 0.7 for the aligned molecule and -0.1 for the anti-aligned molecule, coupled to 〈P4(cos θ)〉 between 0.3 and 0.0. The method to extract the alignment parameters 〈P2(cos θ)〉 and 〈P4(cos θ)〉 directly from the velocity map ion images will be discussed.

Strong field sequential ionization of symmetric-top CH

Synopsis In the interstellar medium polycyclic aromatic hydrocarbon molecules (PAH) are exposed to strong ionizing radation leading to complex organic photochemistry. We investigated these ultrafast fragmentation reactions after ionization of the PAHs phenanthrene, fluorene and pyrene at a wavelength of 30.3 nm using pump probe spectroscopy at a free electron laser. We observe double ionization and afterwards hydrogen abstraction and acetylene loss with characteristic time scales for the reaction processes below one hundred femtoseconds.

In this work, we mainly investigate the NH3 molecular multiphoton ionization process by using the photoelectron velocity map imaging technique. Under the condition of femtosecond laser (wavelength at 800 nm), the photoelectron images are detected. The channel switching and above-threshold ionization (ATI) effect are also confirmed. The kinetic energy spectrum (KES) and the photoelectron angular distributions (PADs) are obtained through the anti-Abel transformation from the original images, and then three ionization channels are confirmed successfully according to the Freeman resonance effect in a relatively low laser intensity region. In the excitation process, the intermediate resonance Rydberg states are C ∼ A 1 ′ 1 (6 + 2 photons process), B ∼ E ″ 1 (6 + 2 photons process) and C ∼ A 1 ′ 1 (7 + 2 photons process), respectively. At the same time, we also find that the photoelectron angular distributions are independent of laser intensity. In addition, the electrons produced by different processes interfere with each other and they can produce a spider-like structure. We also find ac-Stark movement according to the Stark-shift-induced resonance effect when the laser intensity is relatively high.

Nonadiabatic orientation and alignment of hexapole rotational-state-selected methyl bromine $({\mathrm{CH}}_{3}\mathrm{Br})$ molecules is studied. The temporal evolution of the mixed dc field and laser-induced orientation and alignment is measured by a pump-probe method. The temporal orientation and alignment transients are in good agreement with the time-dependent Schr\"odinger equation calculation we perform. We show that the degree of orientation and alignment is strongly dependent on the selected initial rotational state. The temporal transients originating from two different initial states, the $|111\ensuremath{\rangle}$ state and the mixed $|111\ensuremath{\rangle}+|212\ensuremath{\rangle}$ state, are discussed. Our results show a maximum degree of $\ensuremath{\langle}cos\ensuremath{\theta}\ensuremath{\rangle}=\ensuremath{-}0.7$ for the orientation of ${\mathrm{CH}}_{3}\mathrm{Br}$ molecules in the $|111\ensuremath{\rangle}$ state.

Photoelectron images from single ionization of OCS molecules are measured at 800 nm or 400 nm linearly polarized laser pulses (~50 fs) using velocity-map imaging techniques. The resonant ionization of the molecules has been studied by tracing the electron energy shift in the photoelectron energy spectra at different laser intensities, and the contributions of three excited states at certain laser intensities and wavelengths are identified. During the ionization of OCS molecules, two excited states (1Δ and 1∏) are shifted into resonance at different 800 nm laser intensities, and the Rydberg state 4pσ ∏ is involved in the resonance for a 400 nm laser. The demonstration of resonant excitation contribution experimentally will help further the understanding of OCS ionization in strong laser fields.

The backward propagating and side emitted fluorescence during the femtosecond filamentation in air is experimentally investigated in this paper. By comparing the fluorescence emission in the circular and linear polarization states, we find that in the shorter focal length case, the direct ionization of N 2 greatly affects the fluorescence emission behaviors: the fluorescence from N 2 + and N 2 is always stronger in the linear and circular polarization cases, respectively. Based on the observation, the emission mechanism of nitrogen fluorescence emission induced by a femtosecond filament is discussed.

We report experimental results on strong-field laser interact with aligned CH3CI molecules, the ejection of fragments Hn+ dependent on alignment of CH3Cl molecules has been studied. The mechanism of fragmentation builds on the alignment sensitive ionization and dissociation from inner and outer valence orbitals or the excitation process of molecular have been discussed.