Ion Photoelectron Spectroscopy Research Articles

Femtochemistry under scrutiny: Clocking state-resolved channels in the photodissociation of CH3I in the A-band.

Clocking of electronically and vibrationally state-resolved channels of the fast photodissociation of CH3I in the A-band is re-examined in a combined experimental and theoretical study. Experimentally, a femtosecond pump-probe scheme is employed in the modality of resonant probing by resonance enhanced multiphoton ionization (REMPI) of the methyl fragment in different vibrational states and detection through fragment velocity map ion (VMI) imaging as a function of the time delay. We revisit excitation to the center of the A-band at 268 nm and report new results for excitation to the blue of the band center at 243 nm. Theoretically, two approaches have been employed to shed light into the observations: first, a reduced dimensionality 4D nonadiabatic wavepacket calculation using the potential energy surfaces by Xie et al. [J. Phys. Chem. A 104, 1009 (2000)]; and second, a full dimension 9D trajectory surface-hopping calculation on the same potential energy surfaces, including the quantization of vibrational states of the methyl product. In addition, high level ab initio electronic structure calculations have been carried out to describe the CH3 3pz Rydberg state involved in the (2 + 1) REMPI probing process, as a function of the carbon-iodine (C-I) distance. A general qualitative agreement is obtained between experiment and theory, but the effect of methyl vibrational excitation in the umbrella mode on the clocking times is not well reproduced. The theoretical results reveal that no significant effect on the state-resolved appearance times is exerted by the nonadiabatic crossing through the conical intersection present in the first absorption band. The vibrationally state resolved clocking times observed experimentally can be rationalized when the (2 + 1) REMPI probing process is considered. None of the other probing methods applied thus far, i.e., multiphoton ionization photoelectron spectroscopy, soft X-ray inner-shell photoelectron spectroscopy, VUV single-photon ionization, and XUV core-to-valence transient absorption spectroscopy, have been able to provide quantum state-resolved (vibrational) clocking times. More experiments would be needed to disentangle the fine details in the clocking times and dissociation dynamics arising from the detection of specific quantum-states of the molecular fragments.

High-Level Ab Initio Predictions for the Ionization Energy, Bond Dissociation Energies, and Heats of Formation of Vanadium Methylidyne Radical and Its Cation (VCH/VCH+).

The ionization energy (IE) of VCH, the 0 K V-CH/VC-H bond dissociation energies (D0s), and the heats of formation at 0 K (ΔHf0°) and 298 K (ΔHf298°) for VCH/VCH+ are predicted by the wave function-based CCSDTQ/CBS approach. This composite-coupled cluster method includes full quadruple excitations in conjunction with the approximation to the complete basis set (CBS) limit. The contributions of zero-point vibrational energy, core-valence (CV) correlation, spin-orbit coupling, and scalar relativistic corrections are taken into account. The present calculations show that adiabatic IE(VCH) = 6.785 eV and demonstrate excellent agreement with an IE value of 6.774 7 ± 0.000 1 eV measured with two-color laser-pulsed field ionization-photoelectron spectroscopy. The CCSDT and MRCI+Q methods which include CV correlations give the best predictions of harmonic frequencies: ω2 (ω2+) (bending) = 534 (650) and 564 (641) cm-1 and the V-CH stretching ω3 (ω3+) = 835 (827) and 856 (857) cm-1 compared with the experimental values. In this work, we offer a streamlined CCSDTQ/CBS approach which shows an error limit (≤20 meV) matching with previous benchmarking efforts for reliable IE and D0 predictions for VCH/VCH+. The CCSDTQ/CBS D0(V+-CH) - D0(V-CH) = -0.012 eV and D0(VC+-H) - D0(VC-H) = 0.345 eV are in good accord with the experimentally derived values of -0.028 4 ± 0.000 1 and 0.355 9 ± 0.000 1 eV, respectively. The present study has demonstrated that the CCSDTQ/CBS protocol can be readily extended to investigate triatomic molecules containing 3d-metals.

Investigating Disjoint Non-Kekulé Diradicals with Quantum Monte Carlo: The Tetramethyleneethane Molecule through the Jastrow Antisymmetrized Geminal Power Wave Function

Disjoint non-Kekulé molecules are diradicals that present two independent radical centers and can violate Hund's rule, according to which the ground state should have triplet spin symmetry. The prototype of this class of systems is the tetramethyleneethane (TME) molecule for which indeed ion photoelectron spectroscopy (IPS) experiments revealed the singlet (1)A state to be more stable than the triplet (3)Bu. In this work we investigate the potential energy curves of the two spin states of TME and of the two anionic states of TME(-) ((2)A and (2)B1) as a function of the torsion of the central dihedral angle, with quantum Monte Carlo methods and a Jastrow Antisymmetrized Geminal Power wave function. Through ab initio geometrical optimizations we study the possible structural interconversions between the states, finding results which are in full agreement with the IPS experimental data.

Photoelectron spectroscopy of aqueous solutions: streaming potentials of NaX (X = Cl, Br, and I) solutions and electron binding energies of liquid water and X-.

The streaming potentials of liquid beams of aqueous NaCl, NaBr, and NaI solutions are measured using soft X-ray, He(I), and laser multiphoton ionization photoelectron spectroscopy. Gaseous molecules are ionized in the vicinity of liquid beams and the photoelectron energy shifts are measured as a function of the distance between the ionization point and the liquid beam. The streaming potentials change their polarity with concentration of electrolytes, from which the singular points of concentration eliminating the streaming potentials are determined. The streaming currents measured in air also vanish at these concentrations. The electron binding energies of liquid water and I(-), Br(-), and Cl(-) anions are revisited and determined more accurately than in previous studies.

Photoelectron spectroscopy of size-selected cluster ions using synchrotron radiation

A new experimental setup for photoelectron spectroscopy of size-selected cluster ions using synchrotron VUV radiation as generated by the Swiss Light Source is presented. An intense positively charged cluster ion beam is produced in a high-intensity magnetron sputter source. The clusters are subsequently mass selected in a sector magnet. To maximize the residence time of the cluster ions in the ionization region of the velocity map imaging spectrometer, the cluster ion beam is decelerated where it crosses the light beam. First experiments on (MoO3)n+ (n = 69 and 59) cluster cations show that the approach is capable of delivering photoelectron spectra of size-selected transition metal cluster ions.

Electronic Spectroscopy of 1-(Phenylethynyl)naphthalene

Recently 1-(phenylethynyl)naphthalene (1-PEN) was suggested to be the primary dimerization product of phenylpropargyl radicals and therefore an important polycyclic hydrocarbon in combustion processes. Here we describe a spectroscopic investigation of a genuine 1-PEN sample by several complementary techniques, infrared spectroscopy, multiphoton ionization (MPI), and threshold photoelectron spectroscopy. The infrared spectrum recorded in a gas cell confirms that 1-PEN is indeed the previously observed dimerization product of phenylpropargyl. The origin of the transition into the electronically excited S1 state lies at 30823 cm(-1), as found by MPI. Considerable vibrational activity is observed, and a number of low-wavenumber bands are assigned to a progression in the torsional motion. Values of 6 cm(-1) (S0) and 17 cm(-1) (S1) were derived for the fundamental of the torsion. In the investigated energy range the excited state lifetimes are in the nanosecond range. Spectra of the 1-PEN/Ar cluster exhibit a red shift of the electronic origin of 22 cm(-1), in good agreement with other aromatic molecules. A threshold photoelectron spectrum recorded using synchrotron radiation yields an ionization energy of 7.58 eV for 1-PEN. An excited electronic state of the cation is found at 7.76 eV, and dissociative photoionization does not set in below 15 eV.

Laser Multiphoton Ionization of Tetrakis(dimethylamino)ethylene

The tetrakis(dimethylamino)ethylene (TDAE) molecule possesses the lowest known molecular ionization potential (<5.4 eV) and exhibits an intense Rydberg series between the first and second ionization limit (∼14 eV). The ionization of TDAE using multiphoton ionization photoelectron spectroscopy was carried out using laser light at a variety of wavelengths with a hemispherical energy analyzer. Interestingly, photoelectron signal due to direct two-photon ionization was not seen, rather ionization from a fluorescent charge-transfer state located ∼2.5 eV below the ionization limit was evident and in general agreement with a previous study. In addition, a second intense peak exists corresponding to thermal energy electrons. Measurements of the angular distribution for the electrons due to photoionization from the intermediate state are peaked along the electric field vector of the laser and the thermal electrons direction is independent of this angle. From this, we propose that the thermal peak is most likely due to thermionic emission initiated through excitation of a known long-lived Rydberg state at ∼6.5 eV. Alternately, we speculate that excitation leading to thermionic emission could result from a "collective" excitation mechanism.

Ultrafast Internal Conversion of Aromatic Molecules Studied by Photoelectron Spectroscopy using Sub-20 fs Laser Pulses

This article describes our recent experimental studies on internal conversion via a conical intersection using photoelectron spectroscopy. Ultrafast S2(ππ*)–S1(nπ*) internal conversion in pyrazine is observed in real time using sub-20 fs deep ultraviolet pulses (264 and 198 nm). While the photoelectron kinetic energy distribution does not exhibit a clear signature of internal conversion, the photoelectron angular anisotropy unambiguously reveals the sudden change of electron configuration upon internal conversion. An explanation is presented as to why these two observables have different sensitivities to internal conversion. The 198 nm probe photon energy is insufficient for covering the entire Franck-Condon envelopes upon photoionization from S2/S1 to D1/D0. A vacuum ultraviolet free electron laser (SCSS) producing 161 nm radiation is employed to solve this problem, while its pulse-to-pulse timing jitter limits the time resolution to about 1 ps. The S2–S1 internal conversion is revisited using the sub-20 fs 159 nm pulse created by filamentation four-wave mixing. Conical intersections between D1(π−1) and D0(n−1) and also between the Rydberg state with a D1 ion core and that with a D0 ion core of pyrazine are studied by He(I) photoelectron spectroscopy, pulsed field ionization photoelectron spectroscopy and one-color resonance-enhanced multiphoton ionization spectroscopy. Finally, ultrafast S2(ππ*)–S1(ππ*) internal conversion in benzene and toluene are compared with pyrazine.

HIGH-RESOLUTION PHOTOELECTRON SPECTROSCOPY BY SPECTRAL PHASE STEP MODULATION

Femtosecond-induced resonance-enhanced multi-photon ionization photoelectron spectroscopy (REMPI-PES) has the drawback of low spectral resolution due to broadband spectrum of the laser. In this paper, we theoretically demonstrate that high-resolution REMPI-PES can be achieved by shaping the femtosecond laser pulse with spectral phase step modulation. Our results show that, by manipulating the phase step position and the modulation depth, a narrowband peak or hole in the photoelectron spectrum can be observed, and the position of the narrowband peak or hole is correlated with the eigenenergy of the excite state. Therefore, both high-resolution REMPI-PES and the excited state structure can be obtained by observing the narrowband peaks or holes.

High-resolution resonance-enhanced multiphoton-ionization photoelectron spectroscopy of Rydberg states via spectral phase step shaping

Femtosecond-induced resonance-enhanced multi-photon ionization (REMPI) photoelectron spectroscopy of the Rydberg states is faced with low spectral resolution due to the broadband laser spectrum. In this paper, we theoretically demonstrate that a high-resolution (2 + 1) REMPI photoelectron spectrum of the Rydberg states in a sodium (Na) atom can be obtained by shaping a femtosecond laser pulse with a spectral phase step modulation. Our results show that, by using a phase-shaped femtosecond laser pulse, some narrowband holes or peaks in the REMPI photoelectron spectrum can be created, and the positions of these holes or peaks are correlated with the eigenenergies of the Rydberg states. Thus, by observing these holes or peaks in the REMPI photoelectron spectrum, both the high-resolution REMPI photoelectron spectrum and the energy-level diagram of the Rydberg states can be obtained.

Control of resonance enhanced multi-photon ionization photoelectron spectroscopy by phase-shaped femtosecond laser pulse

In this paper, we theoretically demonstrate that the (2+1+1) resonance enhanced multi-photon ionization photoelectron spectroscopy in sodium atom can be effectively controlled by shaping femtosecond laser pulse with a π phase step modulation in weak laser field, involving its total photoelectron energy, maximal photoelectron intensity, and spectroscopic bandwidth. Our results show that the total photoelectron energy can be suppressed but not enhanced, the maximal photoelectron intensity can be enhanced and also suppressed, and the photoelectron spectroscopy can be tremendously narrowed. These theoretical results can provide a feasible scheme to achieve the high-resolution photoelectron spectroscopy and study the excited state structure in atomic and molecular systems.

Selective excitation of resonance-enhanced multiphoton-ionization photoelectron spectroscopy via a cubic phase modulation

Resonantly enhanced multiphoton ionization photoelectron spectroscopy (REMPI-PS) has proven to be a well-established tool to study the excited-state structure. However, femtosecond-induced REMPI-PS suffers from poor selectivity between neighboring excited states due to the large spectral bandwidth. In this paper, we present a feasible scheme to realize the selective excitation of (2+1) REMPI-PS in sodium atoms. We show that, by shaping the femtosecond laser pulse with a cubic phase modulation, a high-resolution (2+1) REMPI-PS is obtained, and the (2+1) REMPI-PS signal from one excited state is enhanced while that from the other excited state is effectively suppressed. Furthermore, we explain the physical control mechanism of the selective excitation of (2+1) REMPI-PS by considering the two-photon power spectrum.

Resonantly Enhanced Multiphoton Ionization and Zero Kinetic Energy Photoelectron Spectroscopy of Chrysene: A Comparison with Tetracene

We report the electronic and vibrational spectroscopy of chrysene using resonantly enhanced multiphoton ionization (REMPI) and zero kinetic energy (ZEKE) photoelectron spectroscopy. As an isomer of tetracene, chrysene contains a kink in the middle of the four fused hexagonal rings, which complicates not just the symmetry but, more importantly, the molecular orbitals and hence vibronic transitions. Incidentally, the two nearby electronically excited states of chrysene have the same symmetry, and vibronic coupling introduces no out-of-plane vibrational modes. As a result, the REMPI spectrum of chrysene contains essentially only in-plane ring deformation modes, similar to that of tetracene. However, density functional calculations using gaussian even after the inclusion of vibronic coupling can only duplicate the observed REMPI spectrum in a qualitative sense, and the agreement is considerably worse than our recent work on a few pericondensed polycyclic aromatic hydrocarbons and on tetracene. The ZEKE spectrum of chrysene via the origin band of the intermediate electronic state S(1), however, can be qualitatively reproduced by a straightforward Franck-Condon calculation. The ZEKE spectra from vibrationally excited states of the S(1), on the other hand, demonstrate some degree of mode selectivity: the overall intensity of the ZEKE spectrum can vary by an order of magnitude depending on the vibrational mode of the intermediate state. A scaling factor in the theoretical vibrational frequency for the cation is also needed to compare with the experimental result, unlike tetracene and pentacene.

Photoelectron Imaging Spectroscopy of S1(1B2u π,π*) Benzene via 611n (n = 0−3) Levels

We report resonance-enhanced two-photon ionization photo-electron spectroscopy of jet-cooled benzene via the 6(1)1(n) (n = 0-3) vibronic levels in S(1)((1)B(2u) π,π*) using a nanosecond UV laser and photoelectron imaging. The best energy resolution (ΔE/E) was 0.7%. The photoelectron spectrum from the S(1) 6(1)1(3) level (E(vib) = 3284 cm(-1)) in the channel three region exhibited a clear signature of intramolecular vibrational redistribution (IVR). The spectral features were consistent with picosecond zero kinetic energy photoelectron (ZEKE) spectra reported by Smith et al. [ J. Phys. Chem. 1995, 99, 1768]. The photoelectron angular anisotropy parameter β(2) was found to be negative in ionization from the 6(1)1(n) (n = 0-3) levels with photoelectron kinetic energies up to 5000 cm(-1). No influence of a shape resonance was identified.

Coherent phase control of (2+1) resonantly enhanced multiphoton ionization photoelectron spectroscopy

In this work, we report a theoretical study of the coherent phase control of typical (2+1) resonantly enhanced multiphoton ionization photoelectron spectroscopy (REMPI-PES) in an atomic system. By properly designing the spectral phase of a femtosecond pulse, we can realize the manipulation of the photoelectron energy, the photoelectron spectral bandwidth and the photoelectron intensity at a certain kinetic energy. Moreover, we can also obtain a high-resolution photoelectron spectrum and fine energy-level structure of the excited states for a complicated quantum system in spite of the broad-width spectrum of the femtosecond pulse. The theoretical results have promising applications for investigating the molecular structure and understanding the laser-induced ionization dynamics.

FAR-INFRARED SPECTROSCOPY OF CATIONIC POLYCYCLIC AROMATIC HYDROCARBONS: ZERO KINETIC ENERGY PHOTOELECTRON SPECTROSCOPY OF PENTACENE VAPORIZED FROM LASER DESORPTION

The distinctive set of infrared (IR) emission bands at 3.3, 6.2, 7.7, 8.6, and 11.3{\mu}m are ubiquitously seen in a wide variety of astrophysical environments. They are generally attributed to polycyclic aromatic hydrocarbon (PAH) molecules. However, not a single PAH species has yet been identified in space, as the mid-IR vibrational bands are mostly representative of functional groups and thus do not allow one to fingerprint individual PAH molecules. In contrast, the far-IR (FIR) bands are sensitive to the skeletal characteristics of a molecule, hence they are important for chemical identification of unknown species. With an aim to offer laboratory astrophysical data for the Herschel Space Observatory, Stratospheric Observatory for Infrared Astronomy, and similar future space missions, in this work we report neutral and cation FIR spectroscopy of pentacene (C_22H_14), a five-ring PAH molecule. We report three IR active modes of cationic pentacene at 53.3, 84.8, and 266{\mu}m that may be detectable by space missions such as the SAFARI instrument on board SPICA. In the experiment, pentacene is vaporized from a laser desorption source and cooled by a supersonic argon beam. We have obtained results from two-color resonantly enhanced multiphoton ionization and two-color zero kinetic energy photoelectron (ZEKE) spectroscopy. Several skeletal vibrational modes of the first electronically excited state of the neutral species and those of the cation are assigned, with the aid of ab initio and density functional calculations.

Ultrafast Curve Crossing Dynamics through Conical Intersections in Methylated Cyclopentadienes

We explored the curve crossing dynamics of 1,2,3,4-tetramethyl-cyclopentadiene (TMCPD) and 1,2,3,4,5-pentamethyl-cyclopentadiene (PMCPD) upon pi --> pi* excitation to the 1B(2) state using time-resolved, resonance-enhanced multiphoton ionization mass and photoelectron spectroscopy. Upon excitation with a femtosecond laser pulse at 267 nm, the energy relaxation pathway is observed by a time-delayed probe pulse at 400 nm, which ionizes the molecule through Rydberg states that reveal the momentary state of the molecule in the photoelectron spectra. We observe that the initially populated 1B(2) state decays to the 2A(1) surface in 135 fs in TMCPD and 183 fs in PMCPD, followed by a crossing to the ground state 1A(1) surface on 57 and 60 fs time scales for TMCPD and PMCPD, respectively. The spectroscopic signatures of the 2A(1) states are clearly revealed in the two-photon ionization photoelectron spectra. In both systems we observe that the ground states are recovered completely, indicating that no new molecular structures are created on the time scale of the experiment.

Electronic Spectroscopy and Ultrafast Energy Relaxation Pathways in the Lowest Rydberg States of Trimethylamine

Resonance-enhanced multiphoton ionization photoelectron spectroscopy has been applied to study the electronic spectroscopy and relaxation pathways among the 3p and 3s Rydberg states of trimethylamine. The experiments used femtosecond and picosecond duration laser pulses at wavelengths of 416, 266, and 208 nm and employed two-photon and three-photon ionization schemes. The binding energy of the 3s Rydberg state was found to be 3.087 +/- 0.005 eV. The degenerate 3p x, y states have binding energies of 2.251 +/- 0.005 eV, and 3p z is at 2.204 +/- 0.005 eV. Using picosecond and femtosecond time-resolved experiments we spectrally and temporally resolved an intricate sequence of energy relaxation pathways leading from the 3p states to the 3s state. With excitation at 5.96 eV, trimethylamine is found to decay from the 3p z state to 3p x, y in 539 fs. The decay to 3s from all the 3p states takes place with a 2.9 ps time constant. On these time scales, trimethylamine does not fragment at the given internal energies, which range from 0.42 to 1.54 eV depending on the excitation wavelength and electronic state.

Zero kinetic energy photoelectron spectroscopy of tetracene using laser desorption for vaporization

Far infrared (FIR) spectroscopy of polycyclic aromatic hydrocarbons is of particular interest to astrophysics since vibrational modes in this range are representative of the molecular size and shape. This information is hence important for identification of chemical compositions and for modeling of the IR spectrum observed in the outer space. In this work, we report neutral and cation FIR spectroscopy of tetracene vaporized from a laser desorption source. Results from two-color resonantly enhanced multiphoton ionization and two-color zero kinetic energy photoelectron spectroscopy will be presented. Several skeletal vibrational modes of the first electronically excited state of the neutral species and those of the cation are assigned, with the aid of ab initio and density functional calculations. The adiabatic ionization potential is determined to be 55 918 +/- 7 cm(-1). Interestingly, all observed vibrational modes can be rationalized based on a simple Huckle calculation, i.e., by observing the addition or elimination of nodal planes due to electronic excitation and/or ionization. Limited by the Franck-Condon principle and the rigidity of the molecular frame of tetracene, only IR forbidden modes are observed in this work.

3s Rydberg and Cationic States of Pyrazine Studied by Photoelectron Spectroscopy

We have studied 3s(n-1 and pi-1) Rydberg states and D0(n-1) and D1(pi-1) cationic states of pyrazine [1,4-diazabenzene] by picosecond (2 + 1) resonance-enhanced multiphoton ionization (REMPI), (2 + 1) REMPI photoelectron imaging, He(I) ultraviolet photoelectron spectroscopy (UPS), and vacuum ultraviolet pulsed field ionization photoelectron spectroscopy (VUV-PFI-PE). The new He(I) photoelectron spectrum of pyrazine in a supersonic jet revealed a considerably finer vibrational structure than a previous photoelectron spectrum of pyrazine vapor. We performed Franck-Condon analysis on the observed photoelectron and REMPI spectra in combination with ab initio density functional theory and molecular orbital calculations to determine the equilibrium geometries in the D0 and 3s(n-1) states. The equilibrium geometries were found to differ slightly between the D0 and 3s states, indicating the influence of a Rydberg electron on the molecular structure. The locations of the D1-D0 and 3s(pi-1)-3s(n-1) conical intersections were estimated. From the line width in the D1 <-- S0 spectrum, we estimated the lifetime of D1 to be 12 fs for pyrazine and 15 fs for fully deuterated pyrazine. A similar lifetime was estimated for the 3s(pi-1) state of pyrazine by REMPI spectroscopy. The vibrational feature of D1 observed in the VUV-PFI-PE measurement differed dramatically from that in the UPS spectrum, which suggests that the high-n Rydberg (ZEKE) states converging to the D1 vibronic state are short-lived due to electronic autoionization to the D0 continuum.