Narrow-Band Extreme-Ultraviolet Laser Radiation Tunable in the Range 90.5-95 nm

Tunable, narrowband extreme ultraviolet radiation in the range 90.5-95 nm with only limited intensity variations is produced by frequency-tripling ultraviolet light from a frequency-doubled dye laser in a gas-jet of xenon, Acetylene gas is found to be an efficient medium for thirdharmonic generation in this wavelength range as well. The extreme-ultraviolet radiation is applied in a spectroscopic study of the blH~, v = 6-8 andv = 10-12, oIH~, v = 0 and b ~ 1 + r ~ , v = 9 states of molecular nitrogen. From linewidth measurements a value kp = 6 x 10 l° s -1 for the predissociation rate of the b 1H,~, v = 11 state is deduced. PACS: 42.65.Ky, 33.20.Ni, 95.30.Ky In recent years various nonlinear optical wave-mixing techniques have been explored for the generation of coherent Extreme-UltraViolet (XUV) radiation at wavelengths below 100 nm. In particular, sum-frequency mixing processes in krypton and xenon gases were shown to be efficient because of strong resonance-enhancement effects at the atomic twophoton level [1]. Straightforward frequency tripling of the output of frequency-doubled dye lasers also gives access to the XUV, but is in principle less efficient. However, with the availability of powerful Nd:YAG lasers this is no longer a problem. Third-Harmonic Generation (THG) has some practical advantages. Firstly, only a single, frequency-doubled tunable laser is required. Secondly, an on-line calibration spectrum can always be recorded at the visible wavelength yielding a high absolute accuracy at the sextupled frequency in the XUV. Thirdly, continuous scans over larger wavelength ranges in the XUV are possible without changing dyes. In previous studies on THG in gaseous jets of xenon [2-5], the most efficient nonlinear medium for production of radiation in the range 90-100 nm, it was observed that large fluctuations in the conversion efficiency occur because of a resonance-enhancement effect of ns and nd autoionizing Rydberg states in the atom. In the present work it is demonstrated that also this disadvantage can be overcome. By focusing intense UV-pulses at a position shifted from the jet centre in the forward direction a wide wavelength range is accessible with only limited variations in XUV yield. The narrowband XUV radiation is applied for a study of selected excited vibronic states of N 2 in the range 90.595 nm. It is an extension towards shorter wavelengths of previous studies [6, 7] exploring the technique of 1 XUV+ 1 UV two-photon ionization. The absorption spectrum of molecular nitrogen, the most abundant molecule in the earth atmosphere, has been subject of numerous investigations since the early work of Hopfield [8]. The congested and complicated spectrum in the XUV at wavelengths shorter than 100 nm was unraveled by the work of Lefebvre-Brion [9], Dressier [10] and Carroll and Collins [11]. An extensive analysis of perturbations in the vibronic structure of the valence and Rydberg states was performed by Stahel et al. [12], but rotational state dependences of the interactions and heterogeneous perturbations were not yet accounted for. Examples of rotational state-dependent perturbations for a limited number of vibronic states were published by Yoshino et al. [13, 14]. The ongoing research into the spectroscopy of N 2 is partly motivated by the need for accurate data to interprete extreme ultraviolet atmospheric light scattering processes, such as the terrestrial airglow. Also in the atmospheres of the planetary satellites Titan and Triton nitrogen is the major constituent and the analysis of Voyager-1 observations of XUV-emissions from these atmospheres is still in progress [15, 16]. Future space missions carrying higher resolution XUV-spectroscopic equipment are scheduled for flight in 1994 [17]. These applications require accurate data on absorption oscillator strengths [18], electron scattering crosssections [19, 20], excited-state emissions [21], predissociation rates [6], collisional deactivation rates and line positions. The present study contributes to this field with accurate measurement and absolute wavelength calibration of line positions, particularly for the bandhead regions of bands that were less well resolved in previous studies [11, 13]. Spectroscopic analyses of excited states b ~H~, v = 6-8 and 10-12, o 1Hu, v 0 and b t + = 2 ~ , v = 9 of N 2 are presented. From observed spectral linewidths lower limits to the excited-state lifetimes are deduced. 412 W. Ubachs et al. 1 Production of XUV Radiation: Third-Harmonic Generation in Xe and C2H2 Narrowband XUV radiation is produced by frequency tripling light from a frequency-doubled dye laser pumped by an injection seeded Nd:YAG laser. The wavelength range 90.5-94.5 nm can be covered using fluorescein dye in a basic methanol solution; at longer wavelengths rodamine-6G dye is used. Frequency tripling takes place in a free expanding gas jet near the opening orifice (diameter 1 mm) of a homebuilt piezo-electric pulsed valve, based on a design of Proch and Trickl [22]. The conditions for THG are as follows: UV-light intensity 8 MW (40 mJ in 5 ns) focused with a f = 20cm lens in a waist of 10-20gm, resulting in a confocal parameter on the order of the interaction length of 1 mm. In our setup the intensity of the XUV-laser beam cannot be measured by direct counting of XUV photons, as it is overlapped by a UV-laser beam a million times stronger. Instead, the ionization current of a probe gas, for which propylene (C3H6) is chosen, is monitored. The XUV-induced photoionization of propylene was found to be independent of wavelength [23] in the range 105-120 nm and it is assumed here that it remains constant towards shorter wavelengths. The efficiencies for THG are measured for xenon and acetylene as non-linear media. For xenon a pressure of 3 bar is used as backing pressure for the pulsed valve. Under these conditions optical breakdown in the medium is easily induced. This phenomenon results in intense light bursts emanating from the tripling zone; moreover pulses of broadband soft x-ray radiation are found to ionize the molecular beam. Lowering the pressure in the tripling zone, effected by decreasing the travel of the plunger in the pulsed valve, then prevents breakdown to occur. In the pressure regime just below breakdown conditions the most efficient generation of narrowband XUV radiation is observed. In studies, where XUV radiation is applied for spectroscopy, these optimum conditions are usually chosen. The wavelength dependence of the XUV intensity, covering the range of the dye fluorescein, is shown in Fig. 1 for Xe and C 2 H 2. The conversion efficiency curves show that in the present setup the XUV wavelength can be continuously tuned in the relatively wide wavelength range of 90.5-94.3 nm, without changing the dye in the laser and with intensity variations of only 50%. The XUV intensity in Fig. 1 is recorded by averaging over 2 shots per frequency setting; a shot-to-shot noise for the XUV-laser source of 20% may be deduced. The wavelength of 90.5 nm is the shortest that can be generated by sextupling a dye laser pumped by the green output of the Nd:YAG laser. Shorter wavelengths are generated by pumping dyes with the UV output of the Nd:YAG laser, but at the cost of intensity. The observed tripling curve for Xe markedly differs from that in some previous studies [4-5], where clear evidence was seen of resonance-enhanced THG by ns and nd autoionizing Rydberg states between the 2P3/2 and 2P1/3 ionization limits. The generated intensity I3~ in THG is proportional to: I3~ o e( N21X(3)(3co, co, co, co)12F(bAk)I3~, (1) with N the density of the medium and I~ the intensity of the fundamental beam. F(bAk) is the so-called phase-matching integral [2, 3, 24], which strongly depends on the focusing geometry. The third-order nonlinear susceptibility X (3) may be approximated by a single dominant resonance term X(3)(3CO, W, ~ , co) = ~ /°I#[1}(lI#p2)/21#[3)(31#P°) (2) where states li), i = 1,2, 3 ly at the/-photon level and 10) is the atomic ground state. /~ represents the damping rate related to resonance levels in the medium; in the case of xenon only occurring at the three-photon level. There, r~s and nd Rydberg states give rise to resonance enhancement. The generated frequency COxu v = 3co is reabsorbed in the "~.~ 5 1 THG in C2H 2