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A. L. Chakraborty and A. Roy
2.5 Line Selection
Line selection is the most critical step in TDLS. Judicious choice of a spectral line
strongly influences the detection sensitivity for a particular gas, the cross-sensitivity
to interfering species, the ability to detect multiple gases with a single laser, the
ability to operate at elevated pressure and temperature and the the ease of signal
normalization by using the spectral wings. The operating conditions must be taken
into account to ensure that the right kind of laser is selected for a given application.
This is a nuanced aspect of TDLS that deserves careful consideration. Figure 4 shows
the simulated absorption spectra of CO, CO 2 and water vapour for the wavelength
range 1000–6000 nm. The mole fraction (X), pressure (P), temperature (T) and path
length (L) values used for the simulation are typical of the in-field measurement
conditions. The absorption spectrum shows densely packed discrete lines resulting
from the transitions from rotational states of one vibrational level to those of another
vibrational level. The mid-infrared transitions are clearly much stronger than the
near-infrared lines and this is the reason why mid-infrared TDLS offers much higher
detection sensitivity for the same detector noise level. The higher line strength of
the mid-infrared lines is crucial for experiments that involve short path lengths. It
is equally important to take into account the spectral congestion in the mid-infrared
region to minimize spectral interference. It is often necessary to settle for a weaker
absorption line that is well isolated from neighbouring lines. The line spacing of
absorption spectra is different for different molecules. The intensity of a spectral
absorption line is determined by the transition probability and the populations of
the energy levels involved in the transition. Spectral lines corresponding to the more
likely transitions are stronger than the ones that are forbidden. If there are two energy
levels from which transitions to a third level are equally likely, the more intense
spectral line will arise from the level which initially has the greater population. The
ratio of populations of a set of energy levels is given by the Boltzmann distribution.
Figure 5 compares the relative transmission for CO 2 and water vapour corresponding to mid-infrared and near-infrared lines. The mid-infrared line strengths of
CO 2 centered on 4319 nm (2315.19 cm
−1 ) is 7.95 × 10
−19 cm
−1 /(mol-cm
−2 ) while
for the near-infrared lines it is 1.26 × 10
−21 cm
−1 /(mol-cm
−2 ). The R16 line of the
ν 1 + 2ν 2 + ν 3 band at 2003.5 nm (4991.2653 cm
−1 ) circled in green is a good choice
for interrogation because absorption due to water vapour is very weak (line strength
on the order of 10
−23 cm
−1 /mol
−1 cm
−2 ), and it is well isolated from the neighbouring water vapor lines by spectral wings of about 0.5 nm on either side that are
required in direct detection to extract a baseline for signal normalization. Note that
water vapour could prove to be a serious problem if the 2003 nm line was to be
selected. Similarly, the mid-infrared line of CO 2 at 4319.3 nm with a line strength
of 7.95 × 10
−19 cm
−1 /mol
−1 cm
−2 is an excellent choice. A stronger line could well
be chosen in this case because there is no spectral interference from water vapour in
this region.
A. L. Chakraborty and A. Roy
2.5 Line Selection
Line selection is the most critical step in TDLS. Judicious choice of a spectral line
strongly influences the detection sensitivity for a particular gas, the cross-sensitivity
to interfering species, the ability to detect multiple gases with a single laser, the
ability to operate at elevated pressure and temperature and the the ease of signal
normalization by using the spectral wings. The operating conditions must be taken
into account to ensure that the right kind of laser is selected for a given application.
This is a nuanced aspect of TDLS that deserves careful consideration. Figure 4 shows
the simulated absorption spectra of CO, CO 2 and water vapour for the wavelength
range 1000–6000 nm. The mole fraction (X), pressure (P), temperature (T) and path
length (L) values used for the simulation are typical of the in-field measurement
conditions. The absorption spectrum shows densely packed discrete lines resulting
from the transitions from rotational states of one vibrational level to those of another
vibrational level. The mid-infrared transitions are clearly much stronger than the
near-infrared lines and this is the reason why mid-infrared TDLS offers much higher
detection sensitivity for the same detector noise level. The higher line strength of
the mid-infrared lines is crucial for experiments that involve short path lengths. It
is equally important to take into account the spectral congestion in the mid-infrared
region to minimize spectral interference. It is often necessary to settle for a weaker
absorption line that is well isolated from neighbouring lines. The line spacing of
absorption spectra is different for different molecules. The intensity of a spectral
absorption line is determined by the transition probability and the populations of
the energy levels involved in the transition. Spectral lines corresponding to the more
likely transitions are stronger than the ones that are forbidden. If there are two energy
levels from which transitions to a third level are equally likely, the more intense
spectral line will arise from the level which initially has the greater population. The
ratio of populations of a set of energy levels is given by the Boltzmann distribution.
Figure 5 compares the relative transmission for CO 2 and water vapour corresponding to mid-infrared and near-infrared lines. The mid-infrared line strengths of
CO 2 centered on 4319 nm (2315.19 cm
−1 ) is 7.95 × 10
−19 cm
−1 /(mol-cm
−2 ) while
for the near-infrared lines it is 1.26 × 10
−21 cm
−1 /(mol-cm
−2 ). The R16 line of the
ν 1 + 2ν 2 + ν 3 band at 2003.5 nm (4991.2653 cm
−1 ) circled in green is a good choice
for interrogation because absorption due to water vapour is very weak (line strength
on the order of 10
−23 cm
−1 /mol
−1 cm
−2 ), and it is well isolated from the neighbouring water vapor lines by spectral wings of about 0.5 nm on either side that are
required in direct detection to extract a baseline for signal normalization. Note that
water vapour could prove to be a serious problem if the 2003 nm line was to be
selected. Similarly, the mid-infrared line of CO 2 at 4319.3 nm with a line strength
of 7.95 × 10
−19 cm
−1 /mol
−1 cm
−2 is an excellent choice. A stronger line could well
be chosen in this case because there is no spectral interference from water vapour in
this region.