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Two tunable dye lasers have been used to generate coherent radiation at a tunable sum frequency in the vacuum-ultraviolet spectral region. Spectral ranges of ~3500 cm"" 1 around 1895 A and ~ 1200 cm" 1 around 1798 A have been covered. These ranges may easily be extended. Large (~*10 ) enhancements of 1v x sum mixing are seen when 2v x is tuned to a double-quantum allowed transition from the ground state in Sr vapor.The original work by Harris and co-workers 1 on extending nonlinear optical-mixing methods into the vacuum-ultraviolet (vuv) spectral range has raised great interest in such techniques. We have employed tunable dye lasers to take advantage of the resonant enhancements of the nonlinearities in atomic vapors in order to generate easily detectable, tunable, coherent, vuv radiation via third-order sum mixing. With the use of several combinations of dyes, spectrally narrow vuv beams were generated and continuously tuned from 1778 to 1817 A (a range of ~ 1200 cm" 1 ) and from 1833 to 1957 A (a range of ~3500 cm" 1 ). Tunable coherent radiation in this range will be useful for a variety of absorption and resonance fluorescence studies in simple atoms and molecules.The resonant enhancements occur when the input lasers are tuned to double-quantum transitions. With a single dye laser, we have observed that the intensity of third harmonic generation (THG) in Sr vapor increases by many orders of magnitude when the fundamental frequency is tuned to the half-frequency of a two-photon allowed transition. Since only two photons are needed for this resonance, one is free to mix in the light from a second dye laser with no constraint on its frequency, thereby generating tunable output at the frequency 2v 1 + v 2y where v Y is the frequency of the fixed laser at the half-frequency of the double-quantum transition, and v 2 is the frequency of the tunable laser.A block diagram of the experiment is shown in Fig. 1. The Molectron UV-1000 nitrogen laser produces ~ 1-MW, 7.5-nsec long pulses at -15 pulses/sec. The collinear, orthogonally polarized dye laser beams leaving the Glan prism have powers between 15 and 100 kW each. The dye lasers incorporated optics 2 that narrowed the spectral bandwidths to ~0.1 and ~1 cm" 1 , respectively. The beams were focused, via a 33-cm focal-length lens, into the center of a simple Sr vapor cell made of nickel and heated to the 800-900°C range. Helium gas at pressures from 100 to 700 Torr and two |-in. apertures placed 25 cm apart (the approximate length of the hot zone) prevented the Sr vapor from diffusing to, and coating, the Pyrex input and LiF output windows. The output light could be observed either directly or through a McPherson model 225 vuv spectrometer using an EMR 541GX-08-18 solarblind photomultiplier. This detector has no response to direct beams from the dye lasers with A>4500 A.The importance of resonant enhancement was dramatically illustrated when a single, linearly polarized beam from a sodium-fluorescein dye laser was tuned from 5337 to 5710 A. The vuv frequency-tripled li...