Applied Physics B

, Volume 97, Issue 1, pp 199–206

CO2 gas resonance absorption at CO2 laser wavelength in multiple laser coating

Article

DOI: 10.1007/s00340-009-3522-z

Cite this article as:
Jiang, L., Li, L. & Tsai, HL. Appl. Phys. B (2009) 97: 199. doi:10.1007/s00340-009-3522-z

Abstract

Multiple laser beams demonstrate many advantages as energy sources in diamond synthesis. In a reported amazingly-fast multiple laser coating technique, CO2 gas is claimed as the sole precursor or secondary precursor for forming a diamond or diamond-like carbon, which remains poorly understood. The absorption coefficient changes under the irradiation of multiple lasers are one of the keys to resolve the mysteries of multiple laser beam coating processes. This study investigates the optical absorption in CO2 gas at the CO2 laser wavelength. The resonance absorption process is modeled as an inverse process of the lasing transitions of CO2 lasers. The well-established CO2 vibrational-rotational energy structures are used as the basis for the calculations with the Boltzmann distribution for equilibrium states and the three-temperature model for non-equilibrium states. Based on the population distribution, our predictions of the CO2 absorption coefficient changes as a function of temperature are in agreement with the published data.

PACS

78.20.Bh 42.62.Cf 76.20.+q 

Nomenclature

a:

average interatomic spacing

A21:

spontaneous transition probability (rate)

Ej:

energy level

g001:

rotational statistical weights of quantum levels of 001

g100:

rotational statistical weights of vibrational quantum levels of 100

h:

Planck constant

j:

index in the numerator in (6)

j′:

index in the denominator in (6)

kB:

Boltzmann constant

K:

constant in (4)

l:

length of the laser-gas interaction zone

n:

constant in (4)

n0:

populations of the ground state

n1:

populations of the 100 state

n2:

populations of the 010 state

n3:

populations of the 001 state

nabs:

number density of the gas

N:

total number of molecules

Np,abs:

average number of absorbed photons

P(Ψ):

population densities

Pg:

gas pressure

RΠ:

twofold vibrational degeneracy of Π states

T:

temperature

T1:

vibrational temperature of 100

T2:

vibrational temperature of 010

T3:

vibrational temperature of 001

Ttr:

translational temperature

Trot:

rotational temperature

Tc:

characteristic temperature

Z:

partition function

Greek symbols

α:

absorption coefficient

λ:

wavelength

τ:

radiative life time for the transition

σ:

absorption cross section

ν0:

frequency at the line center

νc:

optical broadening collision frequency

Δν0:

full width at half-maximum

ν1:

base frequencies of the harmonic oscillators of symmetric stretch mode

ν2:

base frequencies of the harmonic oscillators of bending mode

ν3:

base frequencies of the harmonic oscillators of asymmetric stretch mode

Copyright information

© Springer-Verlag 2009

Authors and Affiliations

  1. 1.Laser Micro-/Nano-Fabrication Laboratory, School of Mechanical EngineeringBeijing Institute of TechnologyBeijingPeople’s Republic of China
  2. 2.School of International Co-EducationBeijing Institute of TechnologyBeijingPeople’s Republic of China
  3. 3.Laser-Based Manufacturing Laboratory, Department of Mechanical and Aerospace EngineeringMissouri University of Science and Technology (Formerly University of Missouri-Rolla)RollaUSA