High resolution VUV spectroscopy of hydrogen

To allow spectroscopy of atomic hydrogen, a source of Lyman-alpha radiation (wavelength 121.56 nm, resonant with th 1S-2P transition in hydrogen) is needed that is both powerful and narrowband [1].
This radiation is produced in five steps:

i)  narrowband cw light source, tunable around at 730 nm (6 time the Lyman alpha wavelength).
    This source is a Ar+ pumped Ti:Saff laser
ii) pulsed amplification in an excimer-pumped dye amplifier.
iii) frequency doubling to 365 nm in a KDP crystal
iv) post amplificatyion i a single excimer pumped UV dye amplifier stage
v) third-harmonic generation in a phase matched mixture of krypon and argon gas. 

The above scheme leads to tunable light with a band width of about 200 MHz, only slightly larger than the natural linewidth (100 MHz) of the 1S-2P transition in hydrogen
In 1999 we launched a project aiming at improving the bandwidth. The essence of this method is described briefly below.

Frequency doubling of pulsed light inside a cavity.

We are investigating the possiblity to imporve the bandwidth of the above scheme by perferming the second and third harmonic generation inside cavities with sufficiently high finnesse. To this end a cavity is kept at resonance using a reference beam and pulsed light of the same frequency is then coupled in.

A new method was devised in which a pulse of light could be captured in a cavity with essentially 100 % efficiency [2]. We used an input coupler with variable reflectivity


[1].  O.J. Luiten, H.G.C. Werij, I.D. Setija, M.W. Reynolds, T.W. Hijmans,
and J.T.M. Walraven,
Lyman-alpha Spectroscopy of Magnetically Trapped Atomic Hydrogen,
Phys. Rev. Lett. 70, 554-557 (1993).

[2]. B.P.J. Bret, T.L. Sonnemans, and T.W. Hijmans, Capturing a light pulse in a short high-finnesse cavity, submitted to Phys. Rev. A.