Absolute Photo-Ionization Rate Measurements Using Ion Imaging Techniques

B. D. DePaola¹, H. Nguyen¹, R. Bredy¹, H. A. Camp¹, X. Flechard², and Z. Chang¹

1) Department of Physics, Kansas State University, Manhattan, KS 66506
2) LPC CAEN, BCD du Maréchal Juin, 14050 CAEN CEDEX, France

Photo-ionization rate measurements generally suffer from a large uncertainty in the laser intensity since ions created in the waist of the beam are extracted along with ions created outside the waist. Thus a large range of intensit ies may contribute to the measured ion yield. In this work, the ion production region is imaged using an ion optics "zoom lens", as shown below in the figure. A 2-dimensional position-sensitive detector gives x -y information, while time-of-flight gives z-information. The extraction field is sufficiently strong to provide 4π steradian collection angle. The laser intensity characteristics are separately determined, and by comparing these with the spatially resolved ion yield data, accurate determination o f relative ionization rates versus laser intensity can be made. These are made absolute by directly measuring the target density. In the particular case presented here, light from a femtosecond laser is directed into a chamber containing rubidium vapor. The target density is determined by measuring absorption of a separate cw diode laser. The technique is general and can also be used to determine relative ionization rates from laser-excited rubidium.

Figures:

Figure 1: Images of ions created in the beam of a femto-second laser. The three separate stripes
are from data collected for three different paths of the laser. This allowed a direct
measurement of the magnification of the ion lens system.

This work was supported by the Chemical Sciences, Geosciences and Biosciences Division,
Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy.

Submitted to ICAP 2002, July 2002 in Cambridge, MA.

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