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Mini-TOF-MS with Cassegrain Optics For Surface Laser Ablation Studies

This instrument uses the same mass spectrometer 'engine' as our other Mini- TOF-MS, but differs considerably in its front-end configuration, because of the very different application.

Photo of Chamber

The instrument was designed for studying the interaction of a femto-second laser with a sample surface in vacuo, onto which various gases are adsorbed. The interaction of the laser and the sample surface generates ions, and the TOF-MS measures the mass of the liberated ions. The laser operates quasi-continuously at 80MHz, and therefore generates ions quasi-continuously also. The extraction lens is pulsed and samples from this stream at up to ~50kHz, using a pulse counting detection scheme, so that each TOF cycle contains data produced during many laser pulses.

The Cassegrain

Line drawing of cassegrain lens

Kore manufactures a Cassegrain optics that is unique because it has a high numerical aperture, an all-metal construction, is ultra high vacuum compatible, and is designed for extraction of ions from near the optical focus.

The Cassegrain geometry allows a relatively long working distance without unduly limiting the numerical aperture and hence diffraction-limited spot size. As a result it has been possible to fit a three-plate electrostatic ion lens assembly between the optical lens and the sample surface. This collects ions created at the optical focus, focussing and directing them through apertures on the axis of the optical elements so they emerge from the rear of the Cassegrain assembly.

Photo of cassegrain and electrostatic lens

Laser light to be focused is directed through the side of the Cassegrain assembly, being reflected onto the optical axis by an integral 45 degree mirror. Thus both light optics and ion optics can share a common axis, normal to the sample surface, with simultaneous high performance.

Photo of system

We now offer the extraction lens, Cassegrain optics, flight tube, reflectron, ion detector and pre-amplifier as a separate add-on TOF-MS, complete with cable set and a single, 19 inch rack power supply.

Schematic of system

Here is a PDF version of the schematic that allows you to zoom in for easier reading of the detail.

The mounting flange in the system shown is a 114 OD CF, and the extraction lens system is re-entrant into the client chamber to a length of 112mm (can be varied upon request). The lens-to-sample distance is 5mm.


Thursday, March 22, 2007, 10:15

© Kore Technology Limited 2007