Competition between resonance-enhanced multiphoton ionisation and third-harmonic generation in acetylene vapour

Observation of the lowest 1Δu Rydberg state of acetylene by multiphoton ionization spectroscopy

“…13, and later explained in terms of competition with third harmonic generation. 14,15 The absence of the H 1 ⌸ u -X 1 ⌺ g ϩ transition in the present work will be discussed in more detail in Sec. III D below.…”

“…7 In the present work we have re-investigated this 3dϩ4s Rydberg supercomplex of C 2 H 2 and C 2 D 2 by ͑3ϩ1͒ photon REMPI under experimental conditions different from those performed in earlier multiphoton works. [13][14][15][16]24 Indeed, by comparison with earlier gas cell REMPI experiments [13][14][15] our spectra were recorded at a much lower pressure ͑at least 2 to 3 order of magnitude͒ and lower laser intensity ͑of about one order of magnitude͒. The main differences between our conditions and earlier beam experiment conditions 15,16,24 concern both the rotational temperature and the laser intensity.…”

“…6 More recently, resonantly enhanced multiphoton ionization ͑REMPI͒ studies have been carried out on the ungerade Rydberg states of acetylene, by using resonant three-photon excitation followed by one-photon ionization. They allowed for the observation of new electronic symmetries which are inaccessible in one photon absorption [13][14][15][16] ͑see Table I͒. In particular, Ashfold et al 13 reported the discovery of the lowest Rydberg state of 1 ⌽ u symmetry belonging to the 3d complex, by ͑3ϩ1͒ REMPI.…”

Prediction of rotational line intensities in doubly-enhanced four-wave mixing (DEFMI) spectroscopy

“…This case of near double-resonance must be treated in the same manner as in example 1 with the appropriate symmetries of states used in equation (13). The main difference between examples 1 and 2 is that the off-resonant effects are primarily occurring at the two-photon level in the former case and at the three photon-level in the latter case.…”

“…Also the problem encountered in THG, of distortion of line intensities because of near-resonant unobserved states at the three-photon level, does not occur and, furthermore, state-selective studies are possible [10] due to strict selection rules for enhancement at the two-and three-photon levels; finally, the double resonance enhancement and the high directionality of the generated radiation ensures at the same time strong and easily detectable signals. Some limitations to the technique are caused by phase-matching effects [12] and by processes in competition with four-wave mixing, such as reabsorption of the output wave [9,10,13] or multiphoton absorption of the input waves [10,14,15]. However, none of these effects appears to affect the lineshapes significantly, provided that one works with molecular beams (short interaction length) and at low laser powers [10].…”