Resonance-enhanced Multiphoton-ionization Photoelectron 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.

Resonance-enhanced multiphoton-ionization photoelectron spectroscopy by a rectangular amplitude modulation

In this Brief Report, we present a scheme to control the (2+1) resonance-enhanced multiphoton-ionization (REMPI) photoelectron spectrum in cesium (Cs) atom by shaping the femtosecond laser pulse with a rectangular amplitude modulation. We show that the amplitude-shaped laser pulse can induce a hole in the (2+1) REMPI photoelectron spectrum, and the position and width of the hole depend on the amplitude step position and amplitude modulation width. We also show that a high-resolution REMPI photoelectron spectrum and the fine energy-level structure of the excited states can be obtained by observing the holes in the (2+1) REMPI photoelectron spectrum. We believe that these theoretical results can provide a feasible method to study the excited-state or Rydberg-state structure and the laser-induced photoionization dynamics.

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.

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.

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.

Femtosecond resonance-enhanced multiphoton-ionization photoelectron spectrum of ammonia

We have studied the multiphoton dissociation dynamics of the ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{E}}^{\ensuremath{'}}\phantom{\rule{0.2em}{0ex}}^{1}A_{1}^{{}^{\ensuremath{'}}}$ Rydberg state of ammonia $({\mathrm{NH}}_{3})$ on a homebuilt femtosecond pump-probe system by resonance-enhanced multiphoton ionization photoelectron (REMPI-PE) spectroscopy. The highly excited Rydberg state, ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{E}}^{\ensuremath{'}1}\phantom{\rule{0.2em}{0ex}}{A}_{1}^{\ensuremath{'}}$, of ammonia was accessed by two $267\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ pump photons and then ionized by a $401\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ probe pulse delayed in time. The variation of the REMPI-PE spectra of ammonia with pump-probe delay time provides valuable information on the dynamics of the excited intermediate accessed by the pump pulse. We find that the Frank-Condon preferred transition during ionization does not occur for $\mathrm{\ensuremath{\Delta}}{\ensuremath{\upsilon}}_{1}=0$ but for $\mathrm{\ensuremath{\Delta}}{\ensuremath{\upsilon}}_{1}=1$, which implies that the intermediate has a different geometry from the ionic ground state. Different dynamical behavior has been observed for each of the transitions $\mathrm{\ensuremath{\Delta}}{\ensuremath{\upsilon}}_{1}=0,1,2,3$, giving a full temporal description of the excited intermediate state by projection onto the eigenspace of the ionic ground state.

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.

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.

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.