Plasma and Thin Film Analysis

In order to investigate the mass and energy distribution of laser generated plasma species in detail, a quadrupole mass spectrometer with a kinetic energy analyzer is available (Hiden Analytical Inc.). This set up allows the direct measurement of the kinetic energy distributions of positive, negative and neutral plasma species up to 1 keV.

The mass spectroscopy setup enables a better understanding of the species composition (left figure) and their respective energy distribution in the plasma (right figure). Information about the detected positive, negative and neutral species in the laser induced plasma can also be obtained. However, for neutral species only a qualitative analysis is possible to to the unkonwn ionisation cross sections of the arriving species.
 

Combining the mass spectrometer with a Multichannel Scaler system, the arrival time of plasma species with a defined mass and selected kinetic energies can be measured.

A Langmuir probe is one of the most simple and direct approaches to monitor the laser induced plasma by measuring the ion current. It works by inserting a metal probe with variable bias voltage into the plasma. The time of arrival signal of the charged species in the plasma can be directly obtained at each bias voltage as shown in the left figure. This enables to estimate the speed of the plasma. Besides, through the relationship between the magnitude of applied bias voltage and detected current signal, the temperature of the electrons can be determined.

Complementary to mass spectroscopy, we investigate the excited species in the laser induced plasma by emission spectroscopy. The first photo shows the experimental imaging/spectroscopy set-up where an image of the laser induced plasma is projected onto the entrance slid of the monochrometer via the indicated optical beam path. Through the combination of different delay times, we can record a spatially and time resolved emission spectrum of the plasma (top right image). The second image shows a La_0.6Ca_0.4MnO₃ spectrum recorded with the spectrometer set-up between 500 and 520nm. Bottom: excitation lines published in the NIST database for the same wavelenght range.

The plasma imaging set-up is shown in the left image. A high speed gated ICCD camera (Andor New i-star) is used to record the time evolution of the ablated material. The images are recorded for either all light admitted through a quartz window (200nm-1000nm) or for selected wavelengths using an Acousto-optic tunable filter (AOTF, 400-1000 nm). The Brimrose models VA210-0.55-1.0-H and VA210-0.40-0.65-H) has a wavelength resolution of 0.6-3 nm with a manufacturer certified resolution of 1.3 nm at 633 nm. An image of an experimental arrangement (cylindrical Ag target with a target-heater/MS distance of 4cm is shown (Fig. (a)), followed by an all light image of the expanding plasma with (b) and without (c) heater. Images by Yao Xiang and Alejandro Ojeda.

As an example of selected line imaging using an AOTF, the next images show the spatial distributions of Ag and Ar excited neutrals at the same time frame of 1.4µs. The AOTF selected optical excitation lines were 827.35 nm for Ag I and 811.53 nm for Ar I. For each image 100 accumulations were used to increase image resolution. The gradients are normalized to the maximum counts for each image. More details can be found in J. Appl. Phys. 120, 225301 (2016).
 

In addition to the frequency, time and space resolved spectroscopy, we do time, space and frequency resolved imaging. One example is shown in the following animation where the plasma expansion of silver was recorded between the initial impact of the laser beam on the target and 10µsec after with a time resolution of 500nsec for each frame. After approx. 2µsec a rebound of the impinging Ag species from the heater is observed. These rebounded species travel back even as far as the target.