Spectroscopic investigation of laser-initiated low-pressure plasmas in atmospheric gases.
Laser-initiated breakdown of N2, O2, and Ar has been investigated at sub-atmospheric pressures using a pulsed (10-nsec) Nd:yttrium aluminum garnet (Nd:YAG) laser at 1.06 μm. Breakdown thresholds have been determined from 3 to 760 Torr, and spectral analysis of the plasma emission has been carried out at 10 and 125 Torr over the 300–600-nm range. Sharp spectral features arising from highly excited singly and multiply charged ions are reported as well as an underlying continuum at intermediate pressure. Time-resolved measurements indicate a complex kinetic dynamic optical behavior.
Laser-induced plasma formation in Xe, Ar, N(2), and O(2) at the first four Nd:YAG harmonics.
Irradiance values have been measured for the onset of laser-induced plasma formation in Xe, Ar, N(2), and O(2) at pressures from 760 to 25 Torr at wavelengths of 1.064, 0.532, 0.355, and 0.266 microm. These values have been compared with the results of other workers who used similar experimental setups. There is agreement within a factor of 4 when irradiance values are compared and within a factor of 2 when ratios of irradiance values for different gasses are compared. Comparisons among workers who used widely different pulse lengths indicate that the onset of plasma formation is measured better by energy fluence than by irradiance.
The Invariance of the Vacuum is the Invariance of the World
The following theorem is demonstrated: If the vacuum is invariant under the group generated by the space integral of the time component of a local vector current, then the Hamiltonian is invariant also. A similar theorem holds for the group generated by the space integral of a space component of a local axial-vector current.