At the time of their development in the fifties, few people would have imagined how lasers would revolutionize scientific research, engineering and manufacturing, let alone impact entertainment, computer storage and communications. While many different laser designs have been developed, they are all based on the same fundamental principle of stimulated emission. The theoretical basis for stimulated emission was first described by Albert Einstein in 1917, and explains how atoms, when excited to a high energy state, are stimulated by a passing photon to relax. As the atom relaxes a second photon is released with the same wavelength and coherency as the passing photon, effectively amplifying the light. This mechanism allows a laser to generate high-intensity light that is both coherent and monochromatic.
An excimer laser is a type of gas laser where the active laser medium comprises two atoms that do not typically form a molecule, or complex, but are enticed to do so when ionized. This is known as an “excimer” or “excited dimer”. There is some contention as to the preferred name for these lasers, as the strictly correct term exciplex (“excited complex” vs. “excited dimer”) has fallen somewhat into disuse, but we use excimer here as this is the more widely preferred specialist term. The excimer laser can utilize a range of gas types, but the preferred choice for LA-ICP-MS is ArF which produces laser light in the deep ultraviolet (DUV, also known as FUV) at 193nm. Changing the gas mixture changes the fundamental wavelength of the emitted light, as per the following table:
|Xe2*||172 & 175 nm||FUV|
Broadly speaking, the total energy emitted decreases with shorter wavelengths. All RESOlution instruments use ArF at 193nm, but other wavelengths are available on request. In some cases, lasers can be modified to produce a different wavelength as required by changing the gas mixture and the resonator optics.
For the laser to operate the gas in the laser needs to be ionized and excited to a high energy state. In an excimer laser this is achieved with a large electrical discharge. This discharge is not maintained continuously, so all excimer lasers fire in pulses with the maximum frequency being dictated by both the electronics and the gas type and pressure. The discharge is a highly energetic process and an electrical load of several kilowatts must be switched from the capacitor banks and through the electrodes into the chamber. This switching mechanism is typically provided by a thyratron, though some lasers can utilize solid-state switches.
The emission process follows this order: 1) Stable gas mixture comprising monatomic gasses receives high voltage discharge. 2) Monatomic gasses combine to form dimer molecules, which in turn are excited to a higher energy level. 3) Excited dimer molecules form short-lived active laser medium and the laser produces optical emission. 4) Dimer excitations decay, dimers separate, and gas mixture returns to stable monatomic state.
The pulse duration from an excimer laser is typically on the order of 5 – 20 ns. In the literature it is common to refer to this as “nanosecond ablation” or “ns-LA-ICP-MS” or similar. For machining applications excimer lasers can fire at up to hundreds of Hz, but for LA-ICP-MS it is more typical to fire at frequencies less than 20 Hz. It should also be noted that the optical emission from an excimer laser is not polarised. An additional interesting property of some excimer laser designs is that they produce relatively large beam sizes with square or rectangular shapes.
Most material, when exposed to high intensity 193nm laser light will ablate directly to a gas phase. This process brings considerable benefit to LA-ICP-MS, resulting in cleaner ablation craters and minimal elemental fractionation. This clean ablation of material by DUV has been widely utilized in the semiconductor industry, where excimer lasers have been heralded as the primary enabler of high resolution photolithography for the manufacture of silicon chips. In medicine, it has been shown that excimer laser light can cut and ablate tissue in a way superior to lasers of other wavelengths. Most people, however, will be exposed to excimer lasers in the field of corrective eye surgery, where excimer laser sources form the cornerstone of the LASIK process.
DUV laser beams can be steered and manipulated using fused silica optics, which offer excellent transmission with 193nm anti-reflection coatings. 193nm also transmits clearly through N2 and CO2 gasses; however, it is sufficiently energetic to ionize O2 into O3. Asides from being slightly corrosive and a mild irritant, O3 is a considerable absorber of DUV light, which is how the ozone layer provides beneficial protection to the earth. Unfortunately, this means that any optical path open to oxygen in the atmosphere will quickly attenuate the laser beam, and accumulation of O3 can also damage the optical elements. For this reason the optical pathway in all RESOlution instruments is tightly sealed, and continuously purged with nitrogen gas.
At wavelengths below 150nm ultraviolet light is absorbed by all gasses, and the beam path must be evacuated to achieve effective transmission efficiency. This region of the optical spectrum is known as the vacuum ultraviolet, or VUV, region.