Threshold ion-pair production spectroscopy of HCl/DCl: Born–Oppenheimer breakdown in HCl and HCl+ and dynamics of photoion-pair formation

Abstract

Accurate ab initio determination of the adiabatic potential energy function and the Born-Oppenheimer breakdown corrections for the electronic ground state of LiH isotopologues Threshold ion-pair production spectroscopy ͑TIPPS͒ has been applied to two isotopomers, HCl and DCl. From the high-resolution TIPP spectra the ion-pair thresholds of the two molecules have been precisely measured. Combined with the known ionization energy of H͑D͒ and the electron affinity of Cl, the difference between their bond dissociation energies is calculated, and therefore an experimental determination of the effect of Born-Oppenheimer breakdown on the dissociation limit of the ground state potential curve has been obtained. The difference in D e for the two isotopomers was found to be: D e (H-Cl)ϪD e (D-Cl)ϭ3.2Ϯ1.0 cm Ϫ1 . The bond energy for HCl was in agreement with our previous determination, D 0 (H-Cl)ϭ35748.2Ϯ0.8 cm Ϫ1 . These results are compared to a recent study of Born-Oppenheimer breakdown in HCl by Coxon and Hajigeorgiou, where high resolution spectroscopic data was used to fit Born-Oppenheimer breakdown correction terms for the intramolecular potential function. The present study also measured the high resolution spectra for photoion-pair production for HCl and DCl in the threshold region for ion-pair production ͑ϳ86 nm͒. Although there is qualitative agreement between the current results and previous experimental and theoretical work, there are some important differences. The possible mechanism for ion-pair formation in HCl and DCl is discussed in light of these high resolution results.

Heavy Rydberg states

“…It should be noted that in most experiments the ionization field F 2 is pulsed (PFI), thus transforming Rydberg states that are excited field-free into above-saddle point Stark states; it is then important whether evolution from low to high fields through crossings between the Stark manifolds, which are actually avoided crossings in all systems but the hydrogen atom, is diabatic or adiabatic. In the present experiment however, different from earlier TIPPS experiments [10,12,15,50], excitation occurs while the discrimination field F 1 is already present. Therefore all Stark states that may be involved in above-saddle point population loss are at the excitation energy, a situation that resembles adiabatic evolution in PFI.…”

Proton formation dynamics in the REMPI[2+n] process via the F Δ12 and f Δ32 Rydberg states of HCl investigated by three-dimensional velocity mapping

“…͑4͔͒ was first found by Yencha et al 23 in one-photon processes using synchrotron radiation in the energy range 14.4-16.4 eV. Hepburn and co-workers 3,24 found similar results using threshold ion-pair production spectroscopy near 14.4 eV photon energy. The analysis of the results showed that the ion-pair is formed by the predissociation of Rydberg states with an A 2 ⌺ + ion core and nl electrons ͑n=4-6͒ by the diabatic curve of the V 1 ⌺ + ion-pair state.…”

“…The excited states and the dynamics involved in the photodissociation and photoionization of the HCl molecule and the corresponding ion HCl + have been the subject of a large number of spectroscopic and theoretical investigations. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] Studies investigating the resonance enhanced multiphoton ionization ͑REMPI͒ [10][11][12][13][14][15][16][17][18][19][20] have revealed fascinating dynamics involving highly excited Rydberg states above the ionization limit. Particularly, the photodissociation and autoionization as well as the subsequent photodissociation of the highly vibrationally excited molecular ion HCl + following the twophoton excitation to the B 1 ⌺ + state have been of special interest.…”

Velocity Imaging Studies on Ion-Pair Dissociation of CH3Br + hνVUV → CH3+ + Br- as a Function of Wavelength

“…In case of I 2 , vibrational levels as high as v > 600 have been observed by means of resonance enhanced multiphoton ionization . In recent years, high-resolution spectroscopic studies on ion-pair states have been performed to determine precise ion-pair dissociation limits and bond dissociation energies for neutral products. , In these studies, a pulsed DC field was used after a certain delay relative to the excitation laser pulse to induce the threshold ion-pair dissociation. Hepburn and co-workers termed the method as threshold ion-pair production spectroscopy (TIPPS) , and successfully applied it to a number of molecular systems, whereas Donovan and co-workers called it zero ion kinetic energy (ZIKE) 5 spectroscopy because of the exact analogy to the previously known zero electron kinetic energy (ZEKE) spectroscopy.…”