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F Merkt

Publications and source records attributed to F Merkt.

15 recordsLinked to original sources
PubMed2006

Jahn-Teller effect in the methane cation: Rovibronic structure and the geometric phase.

The rovibronic structure in the photoelectron spectrum of CH4 has been assigned in infrared-vacuum ultraviolet double-resonance experiments and analyzed using an effective tunneling-rotation Hamiltonian. Comparison of theoretical and experimental level structures of CH4+ reveals the effect of the geometric phase and establishes that at low energies, the structure and dynamics are governed by a large-amplitude tunneling motion connecting equivalent minima of C(2v) geometry. The rotationless ground state of CH4+ is a tunneling doublet of triply degenerate levels and the lowest level has rovibronic symmetry F2. The adiabatic ionization energy of CH4 is 101753.0(15) cm(-1).

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PubMed2006

Rovibrational photoionization dynamics of methyl and its isotopomers studied by high-resolution photoionization and photoelectron spectroscopy.

High-resolution photoionization and pulsed-field-ionization zero-kinetic-energy photoelectron spectra of CH(3), CH(2)D, CHD(2), and CD(3) have been recorded in the vicinity of the first adiabatic ionization threshold following single-photon excitation from the ground neutral state using a narrow-bandwidth vacuum-ultraviolet laser. The radicals were produced from the precursor molecules methyl-bromide, methyl-iodide, dimethyl-thioether, acetone, and nitromethane by 193 nm excimer photolysis in a quartz capillary and were subsequently cooled to a rotational temperature T(rot) approximately equal to 30 K in a supersonic expansion. Nitromethane was identified as a particularly suitable photolytic precursor of methyl for studies by photoionization and threshold photoelectron spectroscopy. Thanks to the cold rotational temperature reached in the supersonic expansion, the rotational structure of the threshold ionization spectra could be resolved, and the photoionization dynamics investigated. Rydberg series converging on excited rotational levels of CH(3) (+) could be observed in the range of principal quantum number n=30-50, and both rotational autoionization and predissociation were identified as decay processes in the threshold region. The observed photoionization transitions can be understood in the realm of an orbital model for direct ionization but the intensity distributions can only be fully accounted for if the rotational channel interactions mediated by the quadrupole of the cation are considered. Improved values of the adiabatic ionization thresholds were derived for all isotopomers [CH(3): 79 356.2(15) cm(-1), CH(2)D: 79 338.8(15) cm(-1), CHD(2): 79 319.1(15) cm(-1), and CD(3): 79 296.4(15) cm(-1)].

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PubMed2006

Normal-incidence electrostatic Rydberg atom mirror.

A Rydberg atom mirror has been designed and its operational principle tested experimentally. A supersonic expansion containing H atoms moving with a velocity of 720 m/s initially propagates toward a quadrupolar electrostatic mirror. The H atoms are then photoexcited to n=27 Rydberg states with a positive Stark shift and move in a rapidly increasing electric field. The H atom beam is stopped in 4.8 micros, only 1.9 mm away from the photoexcitation spot, and is then reflected back. The reflection process is monitored by pulsed field ionization and imaging.

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PubMed2006

Bending energy level structure and quasilinearity of the X+ 3B1 ground electronic state of NH2+.

The bending level structure of the quasilinear X+ 3B1 ground electronic state of the amidogen cation NH2+ was studied by pulsed-field-ionization zero-kinetic-energy photoelectron spectroscopy using a near-infrared vacuum-ultraviolet two-photon ionization sequence via selected rovibronic levels of the A 2A1 state of NH2. The careful selection of the intermediate levels permitted to optimize the transition intensities to the lowest vibrational levels of the cation in the photoionization step and to overcome the low sensitivity of previously employed single-photon ionization schemes. For the first time, all bending levels of the cationic ground state with quantum numbers upsilon2,lin + < or =4, N+ < or =4, and /K+/ < or =2 could be observed, enabling a detailed characterization of the large-amplitude bending vibration. The rotational structure corresponds to that of an effectively linear molecule in all observed vibrational levels. The bending vibrational structure which shows marked deviations from a harmonic behavior was analyzed in terms of a semirigid bender model. The bending potential function was obtained from a fit to the experimental data. The height of the barrier at the linear geometry and the bond angle at the potential minimum were determined to be 231.8(22) cm(-1) and 152.54(4) degrees , respectively, and all bending levels are located above the maximum of the barrier.

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PubMed2005

High-resolution pulsed-field-ionization zero-kinetic-energy photoelectron spectroscopic study of the two lowest electronic states of the ozone cation O3+.

The pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectrum of jet-cooled O3 has been recorded in the range 101,000-104,000 cm(-1). The origins of the X 1A1-->X+ 2A1 and X 1A1-->A+ 2B2 transitions could be determined from the rotational structure of the bands, the photoionization selection rules, the photoionization efficiency curve, and comparison with ab initio calculations. The first adiabatic ionization energy of O3 was measured to be 101,020.5(5) cm(-1) [12.524 95(6) eV] and the energy difference between the X+ 2A1 (0,0,0) and A+ 2B2 (0,0,0) states was determined to be DeltaT0=1089.7(4) cm(-1). Whereas the X-->X+ band consists of an intense and regular progression in the bending (nu2) mode observed up to v2+=4, only the origin of the X-->A+ band was observed. The analysis of the rotational structure in each band led to the derivation of the r0 structure of O3+ in the X+ [C2v,r0=1.25(2) A,alpha0=131.5(9) degrees ] and A+[C2v,r0=1.37(5) A,alpha0=111.3(38) degrees ] states. The appearance of the spectrum, which is regular up to 102,300 cm(-1), changes abruptly at approximately 102,500 cm(-1), a position above which the spectral density increases markedly and the rotational structure of the bands collapses. On the basis of ab initio calculations, this behavior is attributed to the onset of large-amplitude motions spreading through several local minima all the way to large internuclear distances. The ab initio calculations are consistent with earlier results in predicting a seam of conical intersections between the X+ and A+ states approximately 2600 cm(-1) above the cationic ground state and demonstrate the existence of potential minima at large internuclear distances that are connected to the main minima of the X+ and A+ states through low-lying barriers.

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PubMed2004

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.

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PubMed2004

Potential energy curves of diatomic molecular ions from high-resolution photoelectron spectra. II. The first six electronic states of Xe2 +.

The pulsed-field-ionization zero-kinetic-energy photoelectron spectrum of Xe(2) has been measured between 90 000 and 109 000 cm(-1) following single-photon excitation from the ground neutral state. Transitions to five of the six low-lying electronic states of Xe(2) (+) could be observed. Whereas extensive vibrational progressions were observed for the X0(g) (+)-->I(1/2u), I(3/2g), and II(1/2u) photoelectron transitions, only the lowest vibrational levels of the I(3/2u) and II(1/2g) states could be detected. Unambiguous assignments of the vibrational quantum numbers were derived from the analysis of the isotopic shifts of the vibrational bands and of the intensity distribution and from the modeling of the potential energy curves. Analytical potential energy curves of spectroscopic accuracy (i.e., approximately 1 meV) were determined for all six low-lying electronic states using a global model, which includes the first (charge-induced dipole, proportional to 1/R(4)) member of the long-range interaction series and treats the spin-orbit interaction explicitly. The assumption of an R-independent spin-orbit coupling constant was tested and found to be an excellent approximation.

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PubMed2004

Deflection and deceleration of hydrogen Rydberg molecules in inhomogeneous electric fields.

Hydrogen molecules are excited in a molecular beam to Rydberg states around n=17-18 and are exposed to the inhomogeneous electric field of an electric dipole. The large dipole moment produced in the selected Stark eigenstates leads to strong forces on the H2 molecules in the inhomogeneous electric field. The trajectories of the molecules are monitored using ion-imaging and time of flight measurements. With the dipole rods mounted parallel to the beam direction, the high-field-seeking and low-field-seeking Stark states are deflected towards and away from the dipole, respectively. The magnitude of the deflection is measured as a function of the parabolic quantum number k and of the duration of the applied field. It is also shown that a large deflection is observed when populating the (17d2)1 state at zero field and switching the dipole field on after a delay. With the dipole mounted perpendicular to the beam direction, the molecules are either accelerated or decelerated as they move towards the dipole. The Rydberg states are found to survive for over 100 micros after the dipole field is switched off before being ionized at the detector and the time of flight is measured. A greater percentage change in kinetic energy is achieved by initial seeding of the beam in helium or neon followed by inhomogeneous field deceleration/acceleration. Molecular dynamics trajectory simulations are presented highlighting the extent to which the trajectories can be predicted based on the known Stark map. The spectroscopy of the populated states is discussed in detail and it is established that the N+=2, J=1, MJ=0 states populated here have a special stability with respect to decay by predissociation.

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PubMed2004

Nonhydrogenic effects in the deceleration of Rydberg atoms in inhomogeneous electric fields.

Argon atoms in Stark states at n approximately 25 have been decelerated and accelerated in inhomogeneous electric fields. The acceleration and deceleration behavior can be understood only by considering the adiabatic Landau-Zener dynamics that take place at the avoided crossings between the Stark states and the limited fluorescence lifetimes of approximately 10 micros.

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PubMed2004

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.

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PubMed2004

Potential energy curves of diatomic molecular ions from high-resolution photoelectron spectroscopy. I. The first six electronic states of Ar2+.

High-resolution photoelectron spectroscopic data have been used to determine the potential energy curves of the first six electronic states of Ar2+. The potential energy functions properly include the effects of the long-range interactions and of the spin-orbit interaction and are of spectroscopic accuracy (1-2 cm(-1)) over a wide range of internuclear distances. The total number of adjustable parameters could be reduced to only 12 by truncating the long-range interaction series after the R(-6) term and assuming an R-independent spin-orbit coupling constant. This assumption was verified to be valid to an accuracy of +/-2 cm(-1) over the range of internuclear distances between 3.0 and 4.6 A. The interaction potential proposed by Siska [P. E. Siska, J. Chem. Phys. 85, 7497 (1986)] was generalized to a form that is expected to be sufficiently flexible to describe chemical bonding in other diatomic molecular ions. The potential energy curves are more accurate than the best available ab initio curves by two orders of magnitude and provide quantitative information on dissociation energies and equilibrium internuclear distances. The local maximum between the two potential wells of the I(1/2g) state was determined to lie 62 cm(-1) below the Ar(1S0)+Ar(+)(2P(3/2)) dissociation limit, and the II(1/2g) state is found to be significantly more bound (De=177 cm(-1)) than previously assumed.

Argon↗
PubMed2000

PFI-ZEKE photoelectron spectra of the methane cation and the dynamic Jahn-Teller effect

High resolution pulsed-field-ionization (PFI) zero-kinetic-energy (ZEKE) photoelectron spectroscopy has been used to record the photoelectron spectra of CH4, CDH3, CD2H2 and CD4. The observed extensive progression of rotationally resolved transitions between 100,800 cm-1 and 104,100 cm-1 reveals for the first time the complex energy level structure of the methane cation. The high resolution enabled the determination of accurate values for the adiabatic ionization potentials of the different isotopomers. Based on a simple one-dimensional model for the pseudorotation in the different isotopomers, progress has been made towards the understanding of the Jahn-Teller effect at low energies. The static Jahn-Teller distortion in the ion could be determined directly from the vibrationless photoelectron transition in CD2H2. The analysis of the rotational structure in this spectrum with a rigid rotor model leads to an approximate experimental C2v structure. The dynamics of the other methane isotopomers near the adiabatic ionization potentials is dominated by large amplitude vibrational motions between equivalent structures. The corresponding ground state tunneling motions takes place on a picosecond time scale.

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PubMed1997

Molecules in high Rydberg states.

The unusual properties of high molecular Rydberg states with principal quantum number n>>100 have enabled the development of new research tools for the study of molecules and ions in the gas phase. These tools range from spectroscopic techniques such as zero kinetic energy (ZEKE) photoelectron spectroscopy and mass-analyzed threshold ionization (MATI) spectroscopy to techniques that are suited to the investigation of photodissociation processes, bimolecular reactions, and state-selected ion-molecule reactions. This review summarizes recent progress in the understanding of the properties of high molecular Rydberg states and gives an overview of recent chemical applications of these states.

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