Resonance-enhanced Multiphoton-ionization Photoelectron Spectroscopy Research Articles

High-Resolution Selective Excitation of Resonance-Enhanced Multiphoton-Ionization Photoelectron Spectroscopy by Shaping Femtosecond Laser Pulses

Femtosecond laser-induced resonance-enhanced multiphoton-ionization photoelectron spectroscopy (REMPI-PS) is faced with two drawbacks of low spectral resolution and poor selective excitation due to the broad spectral bandwidth. We propose a scheme to obtain a high-resolution selective excitation of (2+1) REMPI-PS by combining π and cosinusoidal phase modulation. Our theoretical results indicate that the (2+1) REMPI-PS signals related to neighboring excited states can be differentiated from their indistinguishable photoelectron spectra by the π phase modulation, and then their selective excitation can be realized by supplementally adding the cosinusoidal phase modulation. Furthermore, the physical mechanism of the high-resolution selective excitation of (2+1) REMPI-PS is explained by considering the two-photon power spectrum.

Effect of laser spectral bandwidth on coherent control of resonance-enhanced multiphoton-ionization photoelectron spectroscopy

The high-resolution (2 + 1) resonance-enhanced multiphoton-ionization photoelectron spectroscopy (REMPI-PS) can be obtained by measuring the photoelectron intensity at a given kinetic energy and scanning the single π phase step position. In this paper, we further demonstrate that the high-resolution (2 + 1) REMPI-PS cannot be achieved at any measured position of the kinetic energy by this measurement method, which is affected by the laser spectral bandwidth. We propose a double π phase step modulation to eliminate the effect of the laser spectral bandwidth, and show the advantage of the double π phase step modulation on achieving the high-resolution (2 + 1) REMPI-PS by considering the contributions involving on- and near-resonant three-photon excitation pathways.

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.

Quantum control of femtosecond resonance-enhanced multiphoton-ionization photoelectron spectroscopy

We establish a theoretical model based on perturbation theory to show that the resonance-enhanced multiphoton-ionization photoelectron spectroscopy (REMPI-PS) in cesium (Cs) atom can be effectively enhanced and narrowed by appropriately shaping the femtosecond laser pulse, and the corresponding physical control processes can be well illustrated by considering the inter- and intragroup interferences involving on- and near-resonant three-photon excitation pathways. Consequently, a high-resolution REMPI-PS and fine energy-level diagram of the excited states can be obtained in spite of the broad femtosecond laser spectrum.

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.

Achieving high-resolution photoelectron spectroscopy from a broadband femtosecond laser pulse

Femtosecond-induced resonance-enhanced multiphoton-ionization photoelectron spectroscopy (REMPI-PS) suffers from poor spectral resolution due to the large laser spectral bandwidth. In this paper, we present a scheme to achieve high-resolution REMPI-PS in the potassium atom by shaping the ultrashort femtosecond laser pulse. Our results show that the REMPI-PS bandwidth can be narrowed by several tens of times with a cubic spectral phase modulation, and thus high-resolution REMPI-PS and the fine structure of the energy-level diagram for multiple excited states can be obtained in spite of the broadband femtosecond laser pulse. Furthermore, we explain the physical control mechanism of the narrowband REMPI-PS by considering the two-photon power spectrum.

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.

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.

(3+1) -resonantly enhanced multiphoton ionization-photoelectron spectroscopy of the (3d-4s) supercomplex of acetylene: The geometry of the E state revisited through experiment and theory

(3+1) -resonance enhanced multiphoton ionization-photoelectron spectroscopy (REMPI-PES) has been carried out via various low vibrational levels of the D, F, and E states belonging to the (3d-4s) supercomplex of acetylene. The photoelectron analysis takes into account the Renner–Teller coupling occurring in the ion ground state. In the 74 500–76 500 cm−1 energy range, the coupling between the F Rydberg state and E valence state is strongly revealed through the photoelectron spectra. Moreover, the vibrational analysis of the REMPI-PES spectra enlighten the controversial geometry of the E valence state. They strongly indicate a planar trans-bent geometry of the E valence state, quantitatively confirmed by an ab initio study. This study confirms the tentative frequencies for the ν1 stretching mode (3307 cm−1 for C2H2+, 2572 cm−1 for C2D2+), as well as the frequencies for the bending mode: trans-ν4 (694 cm−1 for C2H2+, 586 cm−1 for C2D2+) and cis-ν5 (775 cm−1 for C2H2+, 569 cm−1 for C2D2+) of the cations.

Photoelectron spectroscopy of ammonia via a fast predissociative state

We report a study on resonance enhanced multiphoton ionization photoelectron spectroscopy (REMPI-PES) involving the fast predissociative A state of ammonia, using nano- and femtosecond lasers. The multiphoton scheme involves (1 + 1), (2 + 2), (2 + 2) + 1 and (2 + 2) + 2 photon processes. We have found a progression of stretching vibrations v1 in the PE spectrum when pumping NH3 A v2 = 0, 1 and 3 as intermediate states. The stretching vibration intensity distributions in the photoelectron spectrum are calculated by using the Chebychev method of the wavepacket propagation. The femtosecond spectrum shows a similar feature to the nanosecond spectrum. However, high laser power also causes band broadening and shifting effect as well as above threshold multiphoton ionization.

Resonance-enhanced multiphoton-ionization photoelectron spectroscopy of even-parity autoionizing Rydberg states of atomic sulphur

Several previously unobserved Rydberg states of the sulphur atom above the lowest ionization threshold are identified and assigned using (2+1) resonance-enhanced multiphoton-ionization photoelectron spectroscopy. All states were accessed by two-photon transitions from either the 3P ground or the 1D excited state, prepared by in situ photodissociation of H2S. The observed states derive from the (2Do)5p and (2Po)4p configurations. For the (2Do)5p 3F and (2Po)4p 3D triplets, extensive photoelectron spectroscopic studies enable a detailed comparison of the autoionization and photoionization rates of these states.

With Courage: The U.S. Army Air Forces in World War II.

Abstract : Resonance-Enhanced Multiphoton Ionization-Photoelectron Spectroscopy. A single time-of-flight (TOF) photo-electron spectrometer is to be used in conjunction with the MPI of jet-cooled molecules. This apparatus will measure energy and single-resolved photoelectron spectra. Such information has been of direct used in measuring the internal state distribution of REMPI-formed ions used in ion-molecule reaction studies. REMPI Spectroscopy of HBr and DBr. The laser induced fluorescence (LIF) technique has been developed to probe ion species. The ions produced directly from REMPI can have their rotational propensity roles in photoionization processes. The ions produced from a reaction between the state selected ions generated by REMPI and other molecules can be observed for studying state to state ion molecule reaction dynamics. State Selected Ion Molecule Reactions. The reactions of ammonia cations with neutral molecule, were conducted using a tandem quadropole mass spectrometer. A new tapole ion trap was proposed to increase vastly our control over and understanding of biomolecular ion-molecular reactions. To make the quadropole/ octapole/quadropole ion trap operational it was necessary to solve the technically challenging problem of interfacing three dissimilar radio frequency (rf) devices.

Resonance-enhanced multiphoton-ionization photoelectron spectroscopy of even-parity Rydberg states of atomic sulfur.

A (2+1) resonance-enhanced multiphoton-ionization photoelectron spectroscopy study of the sulfur atom was performed in the one-photon energy region between 260 and 240 nm. Some 20 previously unobserved even-parity Rydberg states of the sulfur atom are reported, which were accessed by two-photon transitions from the $^{3}$P ground state of the atom, prepared by in situ photodissociation of ${\mathrm{H}}_{2}$S. The ${(}^{4}$${\mathit{S}}^{\mathit{o}}$)np${\mathrm{}}^{3}$P series could be followed up to n=25. This series is perturbed around n=7 by an interloping Rydberg state converging to the first excited ionic limit $^{2}$${\mathit{D}}^{\mathit{o}}$. A two-channel quantum defect theory analysis was performed in order to estimate the composition of the wave functions of the perturbed series members, which is compared with the ionic state branching ratios obtained from photoelectron spectra. This analysis, moreover, enabled the determination of the ionization energy of the lowest ionic state $^{4}$${\mathit{S}}^{\mathit{o}}$ with an improved accuracy as compared to the previously reported value. \textcopyright{} 1996 The American Physical Society.

Two-photon resonance enhanced MPI-PES above the lowest ionization threshold. Observation of the [formula omitted] state of the SH (SD) radical

A (2 + 1) resonance enhanced multiphoton ionization photoelectron spectroscopy (REMPI-PES) study of the hitherto unobserved [ a 1Δ]5 pπ 2Φ Rydberg state of the SH (SD) radical is reported. Despite the fact that this state has an excitation energy which exceeds the lowest ionization energy, ionization is observed to occur preferentially by the absorption of a third photon after the two-photon excitation step, rather than through autoionization. Photoelectron spectra show a large asymmetry in the rotational branching ratios, which is attributed to the high value of Λ + in the ionic state.

Two-Color Picosecond Time-Resolved (2 + 1') Resonance-Enhanced Multiphoton Ionization Photoelectron Spectroscopy on the B1E'' and C' 1A1' States of Ammonia

The picosecond predissociation dynamics of vibronic levels of the B and e’ Rydberg states of ammonia have been investigated in real time by (2 + 1’) two-color pump-probe ionization in combination with photoelectron spectroscopy. The picosecond real-time results are in reasonable agreement with the results obtained from indirect methods using nanosecond excitation. These indirect methods include investigations of the peak intensities and the natural line widths of the rotational lines in the excitation spectra. The photoelectron spectra obtained for (2 + 1) ionization via the B state in NH3 and ND3 are interpreted and shown to allow for an accurate determination of hitherto unknown vibrational frequencies in the ground state of NH3’ (ND3+). For the VI symmetric stretch a frequency of 0.404 f 0.007 eV (0.304 f 0.007 eV) is found, while the frequency of the v4 asymmetric bend vibration has been established as 0.197 f 0.007 eV (0.141 f 0.007 eV). The hydrogen atom fragment, which results from the predissociation of the B and e’ Rydberg states, has been detected in a two-color pump-probe experiment using nanosecond excitation.

Resonance enhanced multiphoton ionization photoelectron spectroscopy and pulsed field ionization via the F 1Δ2(v′=0) and f 3Δ2(v′=0) Rydberg states of HCl

In this paper, we report the first rotationally resolved one- and two-color resonance enhanced multiphoton ionization photoelectron spectroscopy (REMPI-PES) study of the HCl molecule. The agreement between our experimental branching ratios and theoretical investigations is excellent. We also report the first zero kinetic energy pulsed field ionization (ZEKE-PFI) experiments carried out in a ‘‘magnetic bottle’’ electron spectrometer. A direct comparison is made between ZEKE-PFI and REMPI-PES spectra for ionization via several rotational levels of the F 1Δ2(v′=0) and f 3Δ2(v′=0) Rydberg states of HCl. Large differences in both the spin–orbit and rotational branching ratios are found between the ZEKE-PFI and REMPI-PES spectra. These differences can be understood qualitatively on the basis of rotational and spin–orbit autoionization mechanisms.

Resonance-enhanced multiphoton-ionization photoelectron spectroscopy ofnpandnfRydberg states of atomic nitrogen

(2+1) resonance-enhanced multiphoton-ionization photoelectron spectroscopy studies are reported for the metastable $^{2}$${\mathit{D}}_{\mathit{J}}^{\mathit{o}}$ and $^{2}$${\mathit{P}}_{\mathit{J}}^{\mathit{o}}$ nitrogen atomic species presumably produced by photodissociation of the ${\mathrm{N}}_{3}$(X $^{2}\mathrm{\ensuremath{\Pi}}_{\mathit{g}}$) radical. Two-photon transitions to a multitude of doublet Rydberg states of odd parity are observed in the wavelength range 225.5--294.5 nm. This comprehensive coverage follows all np Rydberg states from n=3 to the ground-state ionization limit ${(}^{3}$${\mathit{P}}_{0,1,2}^{\mathit{e}}$) with n=25 being the highest identifiable. Perturbations are observed around n=5 due to an interloping np Rydberg state (n=3) converging to the first excited ionic limit ${(}^{1}$${\mathit{D}}_{2}^{\mathit{e}}$). A limited multichannel-quantum-defect theory (MQDT) analysis was performed to estimate the composition of the wave functions in this bound-bound interaction. In addition, nf Rydberg states from n=4 to 10, and some other weaker quartet states, have also been identified; these, and higher np states (ng8) show a breakdown of LS coupling. Overall, a considerable number of new states have been identified. Photoelectron kinetic-energy scans give information on the ion state distributions from the branching ratios to the $^{3}$${\mathit{P}}_{0,1,2}^{\mathit{e}}$ ground and the $^{1}$${\mathit{D}}_{2}^{\mathit{e}}$ and $^{1}$${\mathit{S}}_{0}^{\mathit{e}}$ excited states of ${\mathrm{N}}^{+}$; these can be compared to calculated values obtained from the MQDT analysis. Core- and non-core-preserving transitions are observed, and in one case almost complete state selection of excited ${(}^{1}$${\mathit{D}}_{2}^{\mathit{e}}$) nitrogen atomic ions is observed. Analysis of the Doppler profiles of the atomic transitions provides evidence for the mechanism which is at play in the formation of N metastable states.

(2+1) resonance-enhanced multiphoton ionization–photoelectron spectroscopy of the OH radical

A (2+1) resonance-enhanced multiphoton ionization–photoelectron spectroscopy (REMPI-PES) study of the OH radical has been carried out in the two-photon energy region between 81 300 and 88 900 cm−1. Translationally and rotationally hot OH radicals are generated via photodissociation of hydrogen peroxide or formic acid. The known D 2Σ− (v′=0–2) and hitherto unobserved 3 2Σ−(v′=0) intermediate states in this region (at 81 815.8 and 87 643.7 cm−1 above the ground state) are shown to possess predominant Rydberg character. From the rotational structure in the REMPI spectrum physical parameters have been derived for these states.

Vibrational branching ratios following two-color excitation of autoionizing n p Rydberg states of H2

Two-color resonantly enhanced multiphoton ionization-photoelectron spectroscopy (REMPI-PES) was used to determine vibrational branching ratios following autoionization of the ungerade npσ 1Σ+u and npπ 1Πu Rydberg states of H2. In this two-step experiment, one laser used to excite the two photon transition to the E,F 1Σ+g, v′=E2, J′=1 state, and a second laser was used to access the autoionizing Rydberg states near the H+2X 2Σ+g, v+=2 ionization limit. Electrons corresponding to the formation of H+2X 2Σ+g, v+=0 and 1 were collected and energy analyzed using a magnetic bottle electron spectrometer. In agreement with the well-known propensity rule for vibrational autoionization, the vibrational branching ratios strongly favor the final ionic state that corresponds to the minimum change in vibrational quantum number. In general, the branching ratio into the v+=1 channel is 94%–96%, while that into the v+=0 channel is 4%–6%; however, two major deviations from this trend were observed for Rydberg states that are perturbed by the 3pπ 1Πu, v=9 and 4pσ 1Σ+u, v=7 states. Although these low n/high v interlopers were not observed in the present work (since their ionization efficiency is near zero), interchannel coupling apparently causes their influence to be felt by nearby Rydberg states, resulting in v+=0 branching ratios as high as 18%. A number of additional studies suggested by these initial results are discussed.

The photoelectron spectrum of ArXe obtained using resonantly enhanced multiphoton ionization

The photoelectron spectrum of the heteronuclear rare gas dimer ArXe was determined using resonantly enhanced multiphoton ionization-photoelectron spectroscopy (REMPI-PES). Three photoelectron peaks are observed corresponding to the production of ArXe+ in the A2Σ+1/2 B2Π3/2, and B2Π1/2 states. The kinetic energies of these peaks are used to determine lower limits for the dissociation energies of the ionic states.