Pulsed Versus CW Lasers for Photoionization

In case of photoionization of the excited level k), the ionization probability per second 

1 Doppler-Limited Absorption and Fluorescence Spectroscopy with Lasers 

The following estimation illustrates under which conditions this maximum ion rate can be realized Typical cross sections for photoionization are Oki 10 cm. If radiative decay is the only deactivation mechanism of the excited level k), we have Rk = Ak 10 s In order to achieve nLjCTH Ak, we need a photon flux nLj 10 cm s of the ionizing laser. With pulsed lasers this condition can be met readily. 

Example 1.16 Excimer laser 100 mJ/pulse, AT = 10 ns, the cross section of the laser beam may be 1 cm - l2 = 2 x 10 cm s . With the numbers above we can reach an ionization probability of Ptk = 2x10 s for all molecules within the laser beam. This gives an ion rate Si that is 2/3 of the maximum rate Sj = ria. 

The advantage of pulsed lasers is the large photon flux during the pulse time AT, which allows the ionization of the excited molecules before they decay by relaxation into lower levels where they are lost for further ionization. Their disadvantages are their large spectral bandwidth, which is generally larger than the Fourier-limited bandwidth An 1/AT, and their low duty cycle. At typical repetition rates of = 10 to 100 s and a pulse duration of AT = 10 s, the duty cycle is only lO- -lO-  

The following estimation illustrates under which conditions this maximum ion rate can be realized  

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