F. S. Tomkins

Vibrational branching ratios following two‐color excitation of autoionizing np 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,Fu20091Σ+g, v’=E2, J’=1 state, and a second laser was used to access the autoionizing Rydberg states near the H+2Xu20092Σ+g, v+=2 ionization limit. Electrons corresponding to the formation of H+2Xu20092Σ+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 ar...

Rotational state distributions from vibrational autoionization of H2

Optical–optical double‐resonance excitation together with electron spectroscopy was used to measure the H+2 rotational state distributions produced by vibrational autoionization of singlet np Rydberg states of H2 . In the two‐color excitation scheme, one laser was used to excite the two‐photon transition to the H2 E, Fu20091∑+g, v’=1, J’=1 state, and a second laser was used to probe single‐photon transitions to the vibrationally autoionized np Rydberg series converging to the Xu20092∑+g, v+=1, N+=1 and N+=3 levels of the ion. The expected P(1)npσ, Q(1)npπ, R(1)np1, and R(1)np3 Rydberg series converging to v+ =1 were observed and assigned, as were several interlopers converging to higher vibrational levels of the ion. Rotationally resolved photoelectron spectra were determined for all of the autoionizing transitions by using a magnetic bottle electron spectrometer. Under the normal assumptions that p waves are ejected and that spin effects are negligible, vibrational autoionization of the upper levels of the P(1...

Double‐resonance studies of rotational autoionization of H2

Two‐color resonantly enhanced multiphoton ionization combined with photoelectron spectroscopy detection was used to study rotational autoionization of np Rydberg states of H2 near the first ionization threshold. In this two‐step experiment, one laser was used to excite a two‐photon transition to the E,Fu20091Σ+g, v’=E0, J’=0–4 levels, and a second laser was used to probe single photon transitions to the rotationally autoionized np Rydberg series coverging to the Xu20092Σ+g, v+=0, N+=1–6 rotational levels of the ion. Assignments were made for a large number of Rydberg states converging to v+=0 and for several interlopers converging to v+=1 and 2. Of the five dipole allowed Rydberg series converging to v+=0 excited from each intermediate J’ level (J’>2), two are allowed to rotationally autoionize in a coupling scheme that assumes ejection of pure p waves in the ionization process and singlet coupling of the spins of the ion core and the outgoing electron. Members of these Rydberg series have large half‐widths, and...

Competition between rotational and vibrational autoionization in H2

Abstract Resonantly enhanced multiphoton ionization-photoelectron spectroscopy was used to study rotational and vibrational autoionization of n p Rydberg states of H 2 between the H 2 + , X 2 Σ g + , ν + = 2, N + = 1 and N + = 3 thresholds. In this two-color experiment, one laser was used to excite the two-photon transition to the E,F 1 Σ g + , ν′ = E2, J′ = 1 intermediate state, and a second laser was used to access the autoionizing Rydberg states. Significant differences in lineshapes were observed for the H 2 + X 2 Σ g + , ν + = 1 and ν + = 2 decay channels. This is the first direct observation of the competition between these autoionization mechanisms, and the results confirm qualitative features of a recent mutichannel quantum defect theory calculation.

Photoionization Dynamics of Excited Molecular States

Resonance Enhanced Multiphoton Ionization (REMPI) utilizes tunable dye lasers to ionize an atom or molecule by first preparing an excited state by multiphoton absorption and then ionizing that state before it can decay. This process is highly selective with respect to both the initial and resonant intermediate states of the target, and it can be extremely sensitive. In addition, the products of the REMPI process can be detected as needed by analyzing the resulting electrons, ions, fluorescence, or by additional REMPI. This points to a number of opportunities for exploring excited state physics and chemistry at the quantum-state-specific level. Here we will first give a brief overview of the large variety of experimental approaches to excited state phenomena made possible by REMPI. Then we will examine in more detail, recent studies of the three photon resonant, four photon (3+1) ionization of H2 via the C 1Πu state. Strong non-Franck-Condon behavior in the photoelectron spectra of this nominally simple Rydberg state has led to the examination of a variety of dynamical mechanisms. Of these, the role of doubly excited autoionizing states now seems decisive. Progress on photoelectron studies of autoionizing states in H2, excited in a (2+1) REMPI process via the E,F 1Σ g + will also be briefly discussed.

Multiphoton studies of molecular autoiohization

Abstract The combination of resonance enhanced multiphoton ionization (REMPI) and photoelectron Spectroscopy (PES) provides a powerful probe of the decay dynamics of highly excited molecular states. In REMPI, an excited state is prepared by the absorption of one or more photons and is then photoionized before it can decay. This process directly probes the photoionization dynamics of the excited resonant intermediate state; furthermore, if an independently tunable laser is used for the ionization step, the resulting double resonance process can selectively excite quasibound autoionizing states in the ionization continuum. In either case, the choice of the intermediate state enables significant control over the excitation process, including which final states can be accessed and which region of internuclear separation is probed. Photoelectron Spectroscopy can be used to characterize the dynamics of the ionization process through measurement of, e.g., photoionization branching ratios and photoelectron angular distributions. Recent REMPI-PES studies on H 2 serve to illustrate the use of multiphoton techniques to access new information on electronic, vibrational, and rotational autoionization mechanisms.

Two‐color studies of autoionizing states in small molecules

Two‐color resonantly enhanced multiphoton ionization (REMPI) is used to study rotational autoionization of molecular hydrogen and nitric oxide. In contrast to conventional, single‐photon techniques, the REMPI technique can be used to study autoionizing levels of H2 that can decay only through very weak interactions such as p‐f mixing and singlet‐triplet mixing. In addition, rotational autoionization that occurs with that change in rotational quantum number greater than 2 can also be examined (i.e., ΔN+≳2). In homonuclear molecules, symmetry considerations require that ΔN+ be even; in heteronuclear molecules, ΔN+ may be even or odd. It is found, however, that NO is in some sense quasi‐homonuclear, and that processes with ΔN+ = even appear to be more efficient than processes with ΔN+ = odd.

Probing Excited States with Multiphoton Ionization

Resonance enhanced multiphoton ionization (REMPI) utilizes tunable dye lasers to ionize an atom or molecule by first preparing an excited state by multiphoton absorption and then ionizing that state before it can decay. This process is highly selective with respect to both the initial and resonant intermediate states of the target, and it can be extremely sensitive. In addition, the products of the REMPI process can be detected as needed by analyzing the resulting electrons, ions, fluorescence, or by additional REMPI. This points to a number of opportunities for exploring excited state physics and chemistry at the quantum-state-specific level. Here we will begin with a brief overview of the large variety of experimental approaches to excited state phenomena made possible by REMPI. Then we will examine in more detail several examples which illustrate some of these approaches: First, we will discuss three photon resonant, four photon (3+1) ionization of H2 via the C1 IIu state. Strong non-Franck-Condon behavior in the photoelectron spectra of this simple Rydberg state has led to the examination of a variety of dynamical mechanisms. Of these, the role of doubly excited autoionizing states now seems decisive.