Pulsed vs CW Lasers for Photoionization

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

The following estimation illustrates whether this maximum ion rate can be realized Typical cross sections for photoionization are CTu 10 cm. If radiative decay is the only deactivation mechanism of the excited level k) we have = Aj 10 s. In order to achieve Ol 7 Aj we need a photon flux Ul 1Q26 cm -s of the ionizing laser. With pulsed lasers this condition can be met readily. 

Excimer laser 100 mJ/pulse, AT = 10 ns, the cross section of the laser beam may be 1 cm Ol2 =2-1025 cm 2 -s l. With the numbers above we can reach an ionization probability of = 2-10 s for all molecules within the laser beam. This gives an ion rate Sj which is 2/3 of the maximum rate Sj = n.  

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 that is generally larger than the Fourier-limited bandwidth At 1/AT and their low duty cycle. At typical repetition rates of fL = 10 to 100 s and a pulse duration of AT = 10 s the duty cycle is only 10 -10  

Assume that the two laser beams LI and L2 for excitation and ionization have the diameter D = 1 cm and traverse a collimated molecular beam with 1 cm cross section perpendicularly. During the pulse time AT = 10 s the distance, travelled by the molecules at the mean velocity v = 500 m/s is d = ATv 5-10 cm. This means that all molecules in the excitation volume of 1 cm can be ionized during the time AT. During the dark time T = 1/fL, however, the molecules travel the distance d = vT 500 cm at fL = 10 s. Therefore, only the fraction 1/500 = 2-10 of all molecules in the absorbing level i) are ionized in a continuous molecular beam. 

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