Ocean Optics LIBS2500plus - Manual (Page 29)

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Chapter 3
Operation
Overview
Laser-Induced Breakdown Spectroscopy (LIBS) is based on the interaction of a laser with a sample to
produce an optical emission spectrum specific to that sample. A pulsed laser focused onto a solid, liquid,
or gas sample vaporizes a small portion of that sample for analysis. The super-heated, ablated material is
transformed into plasma. Plasma is a form of matter in which the original chemical bonds of the substance
are broken apart and the resulting atoms are converted into a mixture of neutral atoms, ions, and electrons.
The atoms and ions within the expanding plasma release some of their energy by emitting light, which
produces a characteristic emission spectrum in the UV, VIS, and NIR spectral range.
Configuration
Set Q-Switch Delay
The Q-Switch enables you to control laser firing from an external device and is the means by which the
OOILIBSplus application controls timing of the ablation process. To facilitate this timing, you must
configure the Q-Switch delay setting to insure that the spectrometers begin acquiring spectral data at the
optimal time within the lifespan of the plasma. Theoretically, this delay is the period of time after the
laser vaporizes the material and the spectrometer begins acquiring data, allowing the plasma sufficient
cooling time. In practice, there are a number of factors that constitute this delay. Upon receiving the
signal to fire, the laser requires a period of time on the magnitude of microseconds to reach maximum
power and fire. Q-Switch delay is the time from when you tell the laser to fire to when it actually fires (or
“lases”).This delay is necessary to allow the laser to reach its maximum inversion energy so that it fires at
full power.
With a dual-pulsed system, there is also the delay time between the firing of the first and second laser
pulse to be considered. Determining the amount of time needed to reach that precise point of time when
the laser has fired and produced the beginnings of plasma is known as "Q-Switch Zeroing". The amount
of time from “Q-Switch Zero” until the spectrometer begins acquiring data is the “Integration Start
Delay”.
The Q-Switch window allows you to set the position of the firing of the laser with respect to the opening
of the electronic aperture. In other words, you are telling the spectrometers when to begin acquiring data
(usually this is a few microseconds after the laser fires). If you do not hear the laser firing, it is most likely
due to an improper setting of the Q-Switch: