Dependence on temperature and pressure
The values of some spectral line parameters depend on environmental temperature and pressure. The database contains the values of these parameters at Tref and Pref of corresponding datasource.
Temperature dependence of the line intensityTo calculate the intensity Sj of jth spectral line at temperatures different from the reference temperature of the datasource, one uses the following expression
Sj(T) = Sj(Tref) · rQ · rB · rE , (1)
where S(Tref) is the intensity of the line j at Tref , rQ is the ratio of total internal partition functions at Tref and T
rQ = Q(Tref)/Q(T) . (2)
The values of the partition functions Q(T) for isotopologues presented in HITRAN are calculated by Fortran program, TIPS.for in the temperature interval from 70 to 3000 K. The values of the partition functions Q(T) for isotopologues not presented in HITRAN are provided by authors of corresponding linelists.
rB accounts for the ratio of Boltzmann populations
rB = exp(−c2·Ejl/T)/exp(−c2·Ejl/Tref) . (3)
rE accounts the effect of stimulated emission
rE = (1 - exp(-c2·WNj/T))/(1- exp(-c2·WNj/Tref)) . (4)
In expessins (3) and (4) Ejl is the lower-state energy of jth line, WNj is the wavennumber of jth line, c2 is the second radiation constant.
Temperature and pressure dependence of the line width
The half-width at half maximum (HWHM) of spectral lineis used on simulation of spectrum functions for building of profile of spectral line (see Line profiles).
The Doppler half-width of the line j is independent of the pressure and calculated by the expression following from expression (2) in previous paragraph
Dj(T) = Dj(Tref)·sqrt(T/Tref) . (5)
The Lorentz half-width of the line j at temperature T, pressure P and partial pressure Pself is calculated as
Lj(T,P) = (Tref/T)Njt · (Ljenv(Tref,Pref) · (P - Pself) + Ljself · Pself) , (6)
where Njt -temperature-dependence exponent defined above.
The air pressure leads to a shift of the line position. This shift is given by expression
Temperature and pressure dependence of the line position
WNj(P) = WNj + Pjshift · P/Pref , (7)
where WNj is the wavenumber of the line j, Pjshift is the pressure shift of the line j at Pref .
The pressure shift should also include a temperature dependence, but that effect is not considered now.