F. Merkt

Determination of the ionization and dissociation energies of the hydrogen molecule.

The transition wave number from the EF (1)Sigma(g)(+)(v = 0, N = 1) energy level of ortho-H(2) to the 54p1(1)(0) Rydberg state below the X(+) (2)Sigma(g)(+)(v(+) = 0, N(+) = 1) ground state of ortho-H(2)(+) has been measured to be 25,209.99756 +/- (0.00022)(statistical) +/- (0.00007)(systematic) cm(-1). Combining this result with previous experimental and theoretical results for other energy level intervals, the ionization and dissociation energies of the hydrogen molecule have been determined to be 124,417.49113(37) and 36,118.06962(37) cm(-1), respectively, which represents a precision improvement over previous experimental and theoretical results by more than one order of magnitude. The new value of the ionization energy can be regarded as the most precise and accurate experimental result of this quantity, whereas the dissociation energy is a hybrid experimental-theoretical determination.

General symmetry selection rules for the photoionization of polyatomic molecules

General rovibronic symmetry selection rules, which are applicable to any molecular symmetry, have been obtained for the photoionization of polyatomic molecules. The use of the molecular symmetry groups leads to a particularly transparent derivation. The photoelectron is characterized by a partial wave expansion in the orbital angular momentum quantum number l. For a given value of l, one-photon electric dipole transitions can only occur between neutral and ionic states that obey the rovibronic symmetry conditions Γrve (neutral) ⊗ Γrve (ion) ⊃ Γ* for l even and Γrve(neutral) ⊗ Γrve(ion) ⊃ Γ(s) for l odd, where Γ(s) and Γ* represent the totally symmetric and the antisymmetric representations, respectively. Combined with the wellknown angular momentum conservation selection rule Δ J = J + - J = l + , l + ½,…, l - ½, l - [EQUATION](where J + and J represent the total angular momentum quantum number of the ionic and the neutral state between which the photoelectronic transition occurs), these symmetry selectio...

Selective field ionization of high Rydberg states: Application to zero-kinetic-energy photoelectron spectroscopy

Sequences of pulsed electric fields have been designed and tested that enable a higher selectivity in the pulsed field ionization of high Rydberg states (n⩾100) than has so far been possible. The enhanced selectivity originates from the permutation of the parabolic quantum numbers n1 and n2 that is induced by a sufficiently rapid inversion of the electric field polarity during a pulse sequence. A reliable procedure, based on numerical simulations of the outcome of pulse field ionization sequences, has been developed to detect and control changes in the parabolic quantum numbers that can occur during a pulse sequence. The procedure can be used to assess under which conditions a clean permutation of the parabolic quantum numbers can be achieved. Unwanted randomization of m, n1 and n2, which reduces the selectivity of the field ionization process, can be avoided by minimizing the time intervals during which the electric field in the pulse sequence is almost zero. The high selectivity reached in the pulsed fi...

HIGH RYDBERG STATES OF ARGON : STARK EFFECT AND FIELD-IONIZATION PROPERTIES

The Rydberg states with principal quantum number located below the ground state of the ion have been studied by pulsed field ionization following single-photon excitation out of the ground state of Ar. The linewidth of the tunable extreme ultraviolet (XUV) laser source used enabled high-resolution measurement of the Stark effect over a wide range of principal quantum numbers and electric field strengths. Particular attention was given to the ionization of high Rydberg states induced by DC and pulsed electric fields. The lowering (expressed in ) of the ionization threshold by DC electric fields is accurately described by when the electric field strength F is expressed in , a result that is in good agreement with predictions of the classical saddle-point model for field ionization. The field-ionization threshold is very sharp: its width decreases from 0.7 to when the DC field strength is reduced from 580 to . Apart from the Stark states located in a very narrow energy range around the saddle-point energy in the potential which are found to ionize very slowly, all Stark states located below the saddle-point energy have lifetimes exceeding several microseconds, whereas those located beyond the saddle-point energy ionize within less than 20 ns. The very slow field ionization that is observed in a narrow range of energies around the classical saddle point can be used to obtain high state selectivity in the pulsed field ionization. The pulsed field-ionization behaviour observed in argon suggests that the rule that is now commonly assumed in the analysis of pulsed-field-ionization (PFI) zero-kinetic-energy (ZEKE) spectra to describe the low-wavenumber onset of a line relative to the position of the corresponding field-free ionization threshold must be used with caution.

Determination of the ionization and dissociation energies of the deuterium molecule (D2)

The transition wave numbers from selected rovibrational levels of the EF (1)Sigma(g) (+)(v=0) state to selected np Rydberg states of ortho- and para-D(2) located below the adiabatic ionization threshold have been measured at a precision better than 10(-3) cm(-1). Adding these wave numbers to the previously determined transition wave numbers from the X (1)Sigma(g) (+)(v=0, N=0,1) states to the EF (1)Sigma(g) (+)(v=0, N=0,1) states of D(2) and to the binding energies of the Rydberg states calculated by multichannel quantum defect theory, the ionization energies of ortho- and para-D(2) are determined to be 124 745.394 07(58) cm(-1) and 124 715.003 77(75) cm(-1), respectively. After re-evaluation of the dissociation energy of D(2) (+) and using the known ionization energy of D, the dissociation energy of D(2) is determined to be 36 748.362 86(68) cm(-1). This result is more precise than previous experimental results by more than one order of magnitude and is in excellent agreement with the most recent theoretical value 36 748.3633(9) cm(-1) [K. Piszczatowski, G. Łach, M. Przybytek et al., J. Chem. Theory Comput. 5, 3039 (2009)]. The ortho-para separation of D(2), i.e., the energy difference between the N=0 and N=1 rotational levels of the X (1)Sigma(g) (+)(v=0) ground state, has been determined to be 59.781 30(95) cm(-1).

Ionization from a double bond: Rovibronic photoionization dynamics of ethylene, large amplitude torsional motion and vibronic coupling in the ground state of C2H4+

Rotationally resolved pulsed-field-ionization zero-kinetic-energy photoelectron spectra of the X-->X+ transition in ethylene and ethylene-d4 have been recorded at a resolution of 0.09 cm(-1). The spectra provide new information on the large amplitude torsional motion in the cationic ground state. An effective one-dimensional torsional potential was determined from the experimental data. Both C2H4+ and C2D4+ exhibit a twisted geometry, and the lowest two levels of the torsional potential form a tunneling pair with a tunneling splitting of 83.7(5) cm(-1) in C2H4+ and of 37.1(5) cm(-1) in C2D4+. A model was developed to quantitatively analyze the rotational structure of the photoelectron spectra by generalizing the model of Buckingham, Orr, and Sichel [Philos. Trans. R. Soc. London, Ser. A 268, 147 (1970)] to treat asymmetric top molecules. The quantitative analysis of the rotational intensity distributions of allowed as well as forbidden vibrational bands enabled the identification of strong vibronic mixing between the X+ and A+ states mediated by the torsional mode nu(4) and a weaker mixing between the X+ and B+ states mediated by the symmetric CH2 out-of-plane bending mode nu7. The vibrational intensities could be accounted for quantitatively using a Herzberg-Teller-type model for vibronic intensity borrowing. The adiabatic ionization energies of C2H4 and C2D4 were determined to be 84 790.42(23) cm(-1) and 84 913.3(14) cm(-1), respectively.

High-resolution millimeter wave spectroscopy and multichannel quantum defect theory of the hyperfine structure in high Rydberg states of molecular hydrogen H2

Experimental and theoretical methodologies have been developed to determine the hyperfine structure of molecular ions from detailed studies of the Rydberg spectrum and have been tested on molecular hydrogen. The hyperfine structure in l=0-3 Rydberg states of H2 located below the X 2Sigmag+(v+=0,N+=1) ground state of ortho H2+ has been measured in the range of principal quantum number n=50-65 at sub-MHz resolution by millimeter wave spectroscopy following laser excitation to np and nd Rydberg states using a variety of single-photon and multiphoton excitation sequences. The np1(1), nd1(1), and the nf1(0-3) Rydberg states were found to be metastable and to have lifetimes of more than 5 micros beyond n=50. Members of other series, such as the nd1(2), nd1(3), and the np1(0) series, were found to have lifetimes of more than 1 mus. Local perturbations induced by low-n Rydberg states belonging to series converging on rovibrationally excited levels of H2+ reduce the lifetimes in narrow ranges of n values. The hyperfine structure is strongly dependent on the value of the orbital angular momentum l. In the penetrating s and p states at n approximately 50 the exchange interaction dominates over the hyperfine interaction and the levels can be labeled by the total electron spin angular momentum quantum number S (S=0 or 1). In the less penetrating d and f Rydberg states, the hyperfine interaction between the core nuclear and electron spins is larger than the exchange interaction and the Rydberg states are of mixed singlet and triplet character. A procedure based on the Stark effect and on the systematic analysis of selection rules and combination differences was developed to determine the orbital and the total angular momentum quantum numbers l and F and to construct an energy map of p and f Rydberg levels between n=54 and 64 with relative positions of an accuracy of better than 1 MHz. Multichannel quantum defect theory (MQDT) was extended to treat the hyperfine structure in molecular Rydberg states and was used to analyze the observed hyperfine structure of the p and f Rydberg states of H2. The frame transformation between the Born-Oppenheimer channels described by the angular momentum coupling scheme (abetaJ) and the asymptotic channels described by the (e[bbetaS+]) coupling scheme was derived and enables an elegant treatment of all intermediate coupling cases. Purely ab initio quantum defect theory reproduced the experimentally determined positions to within 40 MHz for the p levels and 13 MHz for the f levels. By slight adjustments of the quantum defect functions and their energy dependences and by consideration of the p-f interaction, of the singlet-triplet splittings of the f levels, and of the departure of the ionic levels from pure coupling case (bbetaS+), the agreement between theory and experiment could be improved to 600 kHz. By comparing the results of MQDT calculations of the hyperfine structure of f Rydberg levels with those of coupled equations calculations, the frame transformation approximation of MQDT was shown to be accurate to within 300 kHz. The extrapolated ionic hyperfine structure of the X 2Sigmag+(v+=0,N+=1) ionic level corresponds to the ab initio prediciton of Babb and Dalgarno [Phys. Rev. A 46, R5317 (1992)] within the experimental error.

The first rotationally resolved spectrum of CH4

The pulsed-field-ionization (PFI) zero-kinetic-energy (ZEKE) photoelectron spectra of CH4 and CD4 have been recorded in the region 100880–104100 cm−1. From the analysis of the photoelectron spectra the first adiabatic ionization potential of CH4 and CD4 has been determined to be (101773±35) cm−1 and (102210±25) cm−1, respectively. A one-dimensional model for the pseudorotation between three equivalent C2v equilibrium structures shows evidence for a fluxional behavior of CH4+.

High-resolution threshold-ionization spectroscopy of NH3

High-resolution photoionization, zero-kinetic-energy photoelectron and Rydberg-state-resolved threshold-ionization spectra of ammonia and its deuterated isotopomers have been recorded in the region of the lowest vibrational levels (v2+=0,1) of the X+ ground ionic state of NH3+ following single-photon excitation from the ground neutral state using a narrow bandwidth vacuum ultraviolet laser system (bandwidth 0.008 cm−1). The resolution enables the observation of photoionization transitions originating from distinct tunneling components of the ground neutral state and the measurement of the spin-rotational splittings of the ionic energy levels. A new value of the first adiabatic ionization potential of NH3 [I.P.=82 158.751(16) cm−1] has been derived which is more accurate than previous values by almost two orders of magnitude. The photoionization dynamics of NH3 to the lowest vibrational levels of the X+(2A2″) ground state of NH3+ is dominated by the emission of even l photoelectron partial waves, and a s...

The lowest electronic states of Ne2 +, Ar2 + and Kr2 +: comparison of theory and experiment

Non-relativistic configuration interaction (CI) ab initio calculations using large basis sets have been carried out to determine the potential curves of the first electronic states of Ne2 +, Ar2 + and Kr2 +. The spin—orbit interaction was treated assuming that the spin—orbit coupling constant is independent of the internuclear separation (R). For Ar2 +, calculated dissociation energies and equilibrium separations are in good agreement with experimental results. The calculations for Ne2 + suggest that the lowest vibrational level of the I(1/2u) ground state observed by threshold photoelectron spectroscopy by Hall et al. [1995, J. Phys. B: At. molec. opt. Phys., 28, 2435] and assigned to either ν = 0 or ν = 2 actually corresponds to ν = 4. The calculations also predict the I(1/2g) state of Ne2 + and Ar2 + to possess a double-well potential and that of Kr2 + to be repulsive at short range and to only possess a single shallow well at large internuclear separation. The ab initio calculations provide an explanation for the observation made by Yoshii et al. [2002, J. chem. Phys., 117, 1517] that Kr2 + and Xe2 + dissociate after photoemission from the II(1/2u) state to the I(1/2g) state whereas Ar2 + does not.