  We present a high-power tunable deep-ultraviolet (DUV) laser that uses two
consecutive cavity enhanced doubling stages with LBO and CLBO crystals to
produce the fourth harmonic of an amplified homebuilt external cavity diode
laser. The system generates up to 2.75 W of 261.5 nm laser light with a ~2 W
stable steady-state output power and performs second harmonic generation in a
largely unexplored high intensity regime in CLBO for continuous wave DUV light.
We use this laser to perform fluorescence spectroscopy on the
$X^1\Sigma\leftarrow A^1\Pi$ transition in a cold, slow beam of AlCl molecules
and probe the $A^{1} \Pi|v'=0,~J'=1>$ state hyperfine structure for future
laser cooling and trapping experiments. This work demonstrates that the
production of tunable, watt-level DUV lasers is becoming routine for a variety
of wavelength-specific applications in atomic, molecular and optical physics.
