| The purpose of this investigation was to establish an excitation pathway which accessed high-lying, Rydberg series that converged to rovibrational states in the 1Sigma+ electronic state of HCO+. Through the 3 p pi 2pi Rydberg intermediate state, we observe, using double-resonance spectroscopy, Rydberg series converging to the ion. Using the output of a frequency-tripled, pulsed dye laser, we have extended the ionization-detected one-photon absorption spectrum of HCO to include higher excited vibronic levels of the 3 p pi 2pi Rydberg state in the interval from 47 900 to 50 500 cm-1. Included in this spectrum are band systems associated with bending overtones from (030) through (060), as well as cold-band transitions to bend-CO-stretch combinations from (011) through (031). The use of a separate, synchronized, fixed-frequency visible ionization pulse, in a technique that we term assisted REMPI, reveals a number of weak features. We find sequence-band transitions to confirmed states as well as to the higher excited levels, (041) and (070), which fall out of range as cold bands. We find a very weak structure that we assign to the (100)-(000) transition terminating on the CH-stretch fundamental. This band appears in the 3 p pi state at a frequency very close to the value found for o1 in the HCO+ cation. Continuing a trend established in earlier observations, we find that the Renner-Teller coupling of the electronic orbitial and vibrational angular momentum in the 3 p pi 2pi state of HCO varies with bending quantum number, decreasing linearly with v2 in both fundamental as well as bend-CO-stretch combination states. Photoselecting individual vibrational-rotational levels in the intermediate 3 p pi 2pi state, allow us to utilize double resonance to study high-Rydberg series converging to various vibrationally excited states of HCO+. We report precise thresholds for the (040), (050), (060), and (070) states of HCO+ and determine the spectroscopic constants o2, chi22, and g 22. In addition, we determined the anharmonicity associated with CO-stretch (chi23) based on experimentally resolved precise thresholds for the (011) and (031) states of the cation. |
