Two-photon Resonant Three-photon Ionization Research Articles

Nuclear-spin polarimeter using photoelectron signals

It is widely believed that the hyperfine states must be resolved in one way or the other to measure the degree of nuclear-spin polarization of atoms and ions. We theoretically show that the angle-resolved photoelectron signals produced by resonant multiphoton ionization by short and hence broadband two-color laser pulses, which cover all fine-structure and hyperfine manifold of the ground and resonant excited states, can serve as a new type of nuclear-spin polarimeter. Because of the use of time-delayed short laser pulses the proposed method has very high (subnanosecond) time resolution, and does not suffer from the Doppler broadening. Specific results are presented for the two-photon resonant three-photon ionization of hydrogen atoms using 205- and 410-nm laser pulses with picosecond to femtosecond pulse durations. The proposed scheme is immediately applicable to other unstable elements such as ${}^{11}$Be${}^{+}$, ${}^{27}$Mg${}^{+}$, and ${}^{35}$Ca${}^{+}$, etc.

A large aperture magnification lens for velocity map imaging

We have designed and implemented a large aperture electrostatic Einzel lens that magnifies the images of low energy ions or electrons in a standard velocity map imaging apparatus by up to a factor of 5 while allowing the normal use of the apparatus (without blocking any part of the detector). The field strength in the interaction region remains reasonably constant with or without magnification, and the lens can be used in the normal "crush" mode or with any of the different variants of the "slicing" mode. We have characterized the performance of the lens by imaging ion recoil due to two-photon resonant three-photon ionization [(2+1) REMPI] of O((3)P(2)) atoms and by imaging slow NO molecules from the near-threshold photodissociation of the NO-Ar van der Waals complex.

Multiphoton dissociation dynamics of CH3Br

The state-resolved photodissociation dynamics of CH3Br+ cations are studied using a combination of slice imaging and velocity mapping. Parent ions are prepared by (2 + 1) two-photon resonant three-photon ionization of CH3Br through the 5s Rydberg state in the energy region 29,069.8-29,976.0 cm(-1). Analysis of the photoelectron spectra reveals that CH3Br+ ions are produced in the spin-orbit ground electronic state with a wide distribution of vibrational excitation. The photodissociation of CH3Br+ produces CH3+ (X) + Br(2P(1/2))/Br(2P(3/2)) whose product state distribution shows substantial excitation in the umbrella mode of the CH3+ photofragment, especially when concurrently producing the Br(2P(1/2)) channel. The shapes of the photofragment angular distributions clearly show the different dissociation mechanisms in the Br(2P(1/2)) and Br(2P(3/2)) channels. The dissociation dynamics of the 5s Rydberg state of CH3Br are also presented and analyzed.

Three-photon excitation of strontium Rydberg levels

We report here the first measurements of the three-photon excitation spectrum of strontium using a Nd:YAG pumped dye laser in conjunction with an atomic beam apparatus as well as with a thermionic diode detector. The members of the 5s np 1 P 1 and 5s nf 1 F 3 Rydberg series converging to the first ionization threshold are observed. A dominating two-photon resonant three-photon ionization through the 5s6s 1 S 0 level of strontium lying adjacent to the three photon ionization threshold restricted the observation of the higher Rydberg members.

Two-photon resonant three-photon ionization of Be applied to the determination of isotopic abundances

We show that when more than one photons are involved in bound-bound transitions by a broad-band laser for the determination of isotopic abundances, the ac Stark shift may cause the discrimination on ionization between different isotopes for a given atomic species. As an example, we present specific numerical calculations with Be atom in terms of the determination of isotopic abundances.

Structure and dynamics of 3s E′ cyclopropane: A very fluxional multimode Jahn–Teller system

The two-photon absorption spectra of the 3sE′ states of cyclopropane-h6 and cyclopropane-d6 are obtained by two-photon resonant three-photon ionization spectroscopy. Bands are vibronically resolved by virtue of a free-jet expansion. The hot bands of these spectra suggest that the excited state is distorted in both the carbon ring deformation coordinate and the CH2 wag coordinate. These are the lowest e′ modes, and thus their activity can be interpreted as arising from Jahn–Teller interactions. A satisfactory fit of both line positions and intensities is achieved within this model. The dynamics and structure predicted by the Hamiltonian used are described. It is concluded that, in the ground vibronic state, nonadiabatic interactions are small, but the electronically excited molecule is nevertheless very fluxional, sampling a wide range of geometries. Morever, while only minimal barriers to pseudorotation exist in each coordinate individually, there is some locking of the phases of the two modes, so the pseudorotations are not completely independent.

Angular distributions of CH+3 photofragments from CH3I+ prepared by multiphoton ionization

Methyl iodide ions prepared by two-photon resonant three-photon ionization via Rydberg states are photodissociated by a one-photon process to result in the formation of methyl fragment ions by laser irradiation at 335–403 nm. The parallel angular distributions of these fragment ions have rather small anisotropy parameters, suggesting that dissociation is predissociative via the Ã(2E1/2) state of CH3I+.

Two-photon resonant three-photon ionization of the ndD2states of cesium, rubidium, and sodium: Photoelectron angular distributions

Energy-resolved photoelectron angular distributions have been measured for the nd ${}^{2}$D states (n=11--23) in cesium and rubidium atoms for two-photon resonant, three-photon ionization under conditions where the spin-orbit fine structure was not resolved. The photoelectron angular distributions reveal strong fine-structure mixing effects when the time duration of the laser pulse is longer than the time corresponding to the inverse of the fine-structure splitting of the nd states involved. Three-photon ionization photoelectron angular distributions for sodium atoms have been studied both experimentally and theoretically for laser frequencies in which resonance enhancement occurs via the two-photon excited nd ${}^{2}$D states (n=5--9). The laser pulse duration is \ensuremath{\sim}6 ns which is comparable to or less than the fine-structure mixing time for the unperturbed fine-structure nd ${}^{2}$${D}_{5/2}$,3/2 levels. In addition, the high laser powers \ensuremath{\sim}${10}^{8}$ W/${\mathrm{cm}}^{2}$ employed result in alterations of the energy separations of the fine-structure levels (and cause corresponding changes in the mixing times) due to the ac Stark effect. A detailed theoretical analysis is presented and good agreement is obtained with the experimental results.

Rotationally inelastic gas–surface scattering: NH3 from NH3-saturated W(100)

Two-photon resonant three-photon laser ionization spectroscopy is employed to study rotationally inelastic NH3 molecular beam–surface scattering from NH3-saturated W(100). For all surface temperatures (Ts) examined (300 to 650 K), the scattered NH3 rotational distributions are Boltzmann-like. Complete rotational accommodation is observed for Ts≤ 380 K, while for Ts> 380 K, the NH3 rotational temperature is less than Ts. Angular and velocity analysis of the scattered NH3 for Ts= 300 K show cosine and Maxwellian (T=300 K) distributions, respectively. These findings indicate a trapping–desorption scattering mechanism with a surface residence time below 10 μs at Ts= 300 K. Scattering of a nuclear-spin-state (para) enriched NH3 beam prepared using electric deflection techniques indicates that nuclear spin equilibration (ortho–para) is not achieved during the gas–surface encounter.

Multiphoton dissociation/ionization of benzene, pyridine, and diazabenzenes. Detection of an atomic carbon transition

Multiphoton processes in benzene, pyridine, and the diazabenzenes with ∼10 ns, 0.4–2.5 mJ pulsed laser light in the 336–347 nm range, produced resonantly enhanced molecular ions as well as neutral carbon atoms. The carbon atoms were detected by subsequent two-photon resonant (...2p2 1D2→2hν ...2p3p 1P1) three-photon ionization at 340.93 nm. MPI resonances due to vibrations of the 3p Rydberg state appeared in the spectra of pyrazine and pyrimidine, as well as a probable member of the first singlet→triplet band in pyrimidine.

Nuclear spin and inversion state selection of NH 3 via molecular beam electric deflection

A nuclear-spin and inversion state-selected NH 3 beam is produced by coupling molecular beam electric deflection techniques with the extreme rotational cooling of a pulsed supersonic expansion. Two-photon resonant three-photon ionization is used to probe the molecular beam quantum state distribution. The resulting beam is predominantly (> 95%) in a single quantum state (neglecting hyperfine structure), namely J k = 1 1, in the upper inversion state with para nuclear spin symmetry.

A multiphoton ionization study of nh3 rotational relaxation in pulsed NH 3-He supersonic expansions

Two-photon resonant three-photon ionization is employed to study the rotational relaxation of NH 3 in a pulsed supersonic expansion. The different NH 3 nuclear spin modifications (ortho-para) do not interconvert and relax at different rates. Para relaxation is more efficient than ortho relaxation as evidenced by a higher rotational temperature for the ortho spin modification.

Higher excited states of benzene: Symmetry assignments of six gerade Rydberg series by four-photon absorption spectroscopy

We report the observation of six gerade Rydberg series in benzene converging to the first ionization potential, seen in the region from 14 500–18 500 cm−1 as four-photon resonances in the five-photon ionization spectrum of the jet-cooled molecule. Polarization ratios measured for each transition in two- as well as four-photon resonance aid in determining excited state symmetries. We assign a Rydberg series with a quantum defect of 0.76 to higher members of the 3s Rydberg state first seen by Johnson in the near UV two-photon resonant three-photon ionization spectrum of benzene. The observed quantum defect, polarization ratios, and correlation with synchrotron absorption data all strongly support this assignment. Another series, with a quantum defect of 0.24, exhibits a polarization ratio of 4.5±0.5 signifying pure rank four activity. This is the first report of a purely rank four transition in a molecule. We assign this as an excitation to a d2 orbital which when converging to the first ionization potential (the 2E1g cation) gives rise to B1g and B2g Rydberg transitions. Within the D6h point group, these symmetries are allowed only in a four-photon transition. The number of gerade Rydberg series observed as four-photon resonances exceeds those available from the s-(1) and d-(3) bound members of a hydrogenic basis set. The participation of g(l=4) orbitals is indicated as the lowest members of two of the Rydberg series are at the n=5 level.

Multi-Photon Ionization of Excited Atoms in Gaseous Discharge by Ruby Laser Light

Multi-photon ionization of excited states of He or Hg atoms in glow discharge tube by ruby laser light were detected by using the induced variation of discharge current. The probabilities of one-photon, two-photon and three-photon ionizations with an intermediate resonance were also calculated. Laser induced current variation was explained by the calculated value of multi-photon emitted electrons. It was found that two-photon resonant three-photon ionization is dominant in He and three-photon ionization in Hg in case of rather high intense laer light.

Spin polarization of electrons in two-photon resonant three-photon ionization

We present a theory of spin polarization of electrons ejected from atoms by two-photon resonant three-photon ionization. Our treatment includes saturation effects and pays proper attention to the finite rise time of the laser pulse and the non-Lorentzian wings of the laser spectrum. Numerical results for the cesium atoms indicate that due to saturation effects the spin polarization is generally less than its value predicted by perturbation theory. The different saturation behavior of the ionization channels causes a saturation dip to develop in the degree of spin polarization.

Multiphoton dissociation and ionization of nickelocene

In this paper we report the results of an experimental study of collision-free molecular multiphoton dissociation (MPD) and molecular multiphoton ionization (MPI) of nickelocene (NiC 10H 10), induced by the light of a tunable dye laser in the wavelength region 3750–5200 A. The spectral dependence of the ion signal reveals a multitude of narrow (fwhm from <0.5 cm −1 to 1.5 cm −1) intense peaks superimposed on a very weak background (relative amplitude ratio for peaks/background ≈ 10 3). The sharp resonances in the ion signal are attributed, on the basis of spectroscopic analysis, to two-photon resonant three-photon ionization of Ni(I) and to one-photon resonant three-photon ionization of Ni(I), the Ni(I) being produced by MPD of nickelocene. The ion signal in the spectral range 3750–3950 A reveals enhanced continuous background due to MPI of nickelocene. This ion signal spectra, together with studies of the intensity dependence of the overall (nickelocene MPD) - (Ni(I) MPI) processes, as well as the (nickelocene molecular MPI) reaction, reveal four reactive processes. (a) Two-photon molecular MPI for hω ⩾ 3.10 eV photons. (b) Three-photon molecular MPI for hω = 3.10-2.10 eV. (c) Two-photon MPD at hω ⩾ 2.86 eV. (d) Three-photon MPD for hω = 2.8-1.9 eV. The overall dissociation energy of nickelocene (Nicp 2) to give Ni + 2cp was determined to be 5.76 ± 0.60 eV and the two-photon ionization potential of this molecule is 6.29 ± 0.015 eV. Our results provide dynamic evidence concerning a simultaneous “explosive” photodissociation mechanism of Nicp 2 by process (c) and for the dominating role of the dissociative channel, characterized by a branching ratio of ⩾50 in favor of predissociation over autoionization, for process (c) at 6.3–6.6 eV. The MPD processes (c) and (d) are expected to exhibit intramolecular erosion of phase coherence effects. Processes (c) and (d) are of high efficiency ≈0.01 ions/molecule at saturation exhibited at laser power of ≈ 10 8 W cm −2.

Two-photon resonant three-photon ionization with ultrashort pulses

The problem of two-photon resonant three-photon ionization is analysed for the case of ultrashort ionizing pulses. An analytic solution to the problem is obtained and discussed. A simple experiment is proposed which would demonstrate the effect of optical nutation on the photoelectron flux.