Laser measurement of the photodetachment cross section of ${\mathrm{H}}^{-}$ at the wavelength 1064 nm

A method is described to measure photoexcitation cross sections, relying on the expected behavior of the signal in the saturated regime, when excitation is provided by a Gaussian light beam. The method is implemented on a negative ion beam, with a single-mode pulsed Nd:YAG laser, to make a laser measurement of the photodetachment cross section of ${\mathrm{H}}^{-}$, at the wavelength 1064 nm. This cross section is of importance both as a photodetachment cross section of the most elementary negative ion and as a key parameter for the production of fast neutral ${\mathrm{H}}^0$ or ${\mathrm{D}}^0$ atoms, by photodetachment of accelerated anions. The obtained value $4.5\left(6\right)\times {10}^{-21}{\mathrm{m}}^2$ is greater than the one known from older measurements and most ab initio calculations.

Figure 1

The situation considered of two beams crossing at right angles, with a laser beam much narrower than the ion beam, the diameter $L$ of which determines the length of the contributing zone of the laser beam.

Figure 2

Detachment signal as a function of the laser pulse energy plotted along linear and logarithmic scales, which makes the representation of the linear regime look like an exponential and the saturated regime be represented by a straight line. The intercept $E_{\mathrm{S}}$ of this line on the $N=0$ axis is enough to measure the photodetachment cross section. The data represented by light squares (black circles) and interpolated, for the saturated part, by dashes (dots) are for a laser waist $w_0=70.6\left(20\right)\mu \mathrm{m}$ [$80.2\left(20\right)\mu \mathrm{m}$].