The visible absorption spectrum of NO 2 is very dense and irregular, and shows signs of a chaotic frequency and intensity distribution in the higher energy region. The complexity of the spectrum is related to a conical intersection of the potential energy surfaces of the two lowest electronic states. Above the conical intersection strong vibronic interactions lead to hybrid eigenstates, which can be viewed as mixtures of low vibrational levels of the electronically excited state and high vibrational levels of the electronic ground state. As a contribution to the elucidation of the nature of the vibronic bands of NO 2 we have measured high-resolution spectra of a number of vibronic bands in the region between 10 000 and 14 000 cm Ϫ1 by exciting a supersonically cooled beam of NO 2 molecules with a narrow-band Ti:Sapphire ring laser. The energy absorbed by the molecules was detected by a bolometer, and in some cases, laser-induced fluorescence was detected. The hyperfine structure is dominated by the Fermi-contact interaction and the magnitude of this interaction is a direct measure of the ͑electronic͒ composition of the hybrid eigenstates. In the present paper we have restricted our analysis to transitions of K Ϫ ϭ0 stacks. The fine-and hyperfine structure of each rotational transition can be analyzed by using an effective Hamiltonian approach. The very good agreement that is found between the calculated transition strengths and the measured line intensities is evidence that in the spectral region studied, rovibronic interactions play a minor role. The composition of the hybrid eigenstates is compared with ab initio calculations reported in the literature, leading to the conclusion that measurements of the hyperfine structure are a helpful tool in characterizing vibronic bands.

The complexity of the absorption spectrum of NO 2 can be attributed to a conical intersection of the potential energy surfaces of the two lowest electronic states, the electronic ground state of 2 A 1 symmetry and the first electronically excited state of 2 B 2 symmetry. In a previous paper we reported on the feasibility of using the hyperfine splittings, specifically the Fermi-contact interaction, to determine the electronic ground state character of the excited vibronic states in the region just above the conical intersection; 10 000 to 14 000 cm Ϫ1 above the electronic ground state. High-resolution spectra of a number of vibronic bands in this region were measured by exciting a supersonically cooled beam of NO 2 molecules with a narrow-band Ti:Sapphire ring laser. The energy absorbed by the molecules was detected by the use of a bolometer. In the region of interest, rovibronic interactions play no significant role, with the possible exception of the vibronic band at 12 658 cm Ϫ1 , so that the fine-and hyperfine structure of each rotational transition could be analyzed by using an effective Hamiltonian. In the previous paper we restricted ourselves to an analysis of transitions of the K Ϫ ϭ0 stack. In the present paper we extend the analysis to transitions of the K Ϫ ϭ1 stack, from which, in addition to hyperfine coupling constants, values of the A rotational constants of the excited NO 2 molecules can be determined. Those rotational constants also contain information about the electronic composition of the vibronic states, and, moreover, about the geometry of the NO 2 molecule in the excited state of interest. The results of our analyses are compared with those obtained by other authors. The conclusion arrived at in our previous paper that determining Fermi-constants is useful to help characterize the vibronic bands, is corroborated. In addition, the A rotational constants correspond to geometries that are consistent with the electronic composition of the relevant excited states as expected from the Fermi-constants.

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Articles you may be interested inStudy of laser beam propagation in microholes and the effect on femtosecond laser micromachining Laser parameters, which significantly influence laser-material interaction processes, are the wavelength, the energy, and the power density. Additionally, there are parameters, like the pulse length, which also strongly influence processing speed and quality. Studies where different types of lasers have been used indicate that long pulses are beneficial for processing speed. However, when different types of laser systems are used to study the effect of the pulse length, a direct comparison of the results is difficult because the use of different lasers involves a simultaneous variation of other parameters ͑e.g., wavelength͒ as well. In this study a technique of pulse length variation is used in which the pulse length is the only varied parameter and thus enables the desired direct comparison. Pulses with different lengths are sliced out of pulses of a long pulse XeCl excimer laser, keeping all other laser parameters unchanged. Results are shown of hole drilling experiments in 125 m nickel, 25 m aluminum, and 125 m aluminum foil with pulse lengths between 9 and 150 ns. The influence of the pulse length on material processing is discussed in connection with energy and power of the pulses. The experiments show that both for pulses with the same energy and the same power long pulses remove more material than short pulses and, moreover, long pulses can yield higher quality of the drilled holes.

Studies of the influence of pulse length on material processing with different lasers have shown that a long pulse is beneficial for processing speed. In this paper a technique of pulse length variation is used in which the pulse length is the only varied parameter. Pulses between 5 and 150 ns length are sliced out of the 175 ns pulse of a long pulse excimer laser. The beam quality for each sliced pulse length is similar. In this paper the results are shown of hole drilling experiments in 125 µm aluminium foil with pulses of 10 and 100 ns length. The influence of the pulse length on material processing is discussed in relationship with equal energy and equal power density of the pulses. This study shows that in both cases long pulses remove more material than short pulses.