Cavity Ring-Down Spectroscopy
297
5 Applications of Cavity Ring-Down Spectroscopy
CRDS is considered as one of the most popular techniques in analytical spectroscopy.
The enormous sensitivity that can be easily achieved in an optical cavity has allowed
us to detect the species in a variety of environments and consequently has found
applications in a wide range of fields. Here, we have mainly discussed some of its
important applications in trace gas sensing, high-resolution molecular spectroscopy
and medical diagnostics.
5.1 High-Resolution Molecular Spectroscopy
in the Gas-Phase by CRDS
Over the past decades, numerous studies have focused on the high-resolution spectroscopic investigation of several important molecular species and their isotopes such
as N 2 O, NO 2 , CH 4 , H 2 O, D 2 O, OCS etc. by CRDS technique. All these molecular
species have significant impact in environmental chemistry and biomedical science.
However, the Group of Romanini has utilized the CRDS technique for Dopplerbroadened measurements of N 2 O by probing the 6ν 3 − overtone band at 765 nm
[28].
There is another group, Quack and co-workers who have demonstrated the highresolution monitoring of several overtone and combination bands of CH 4 , chloroform
and N 2 O in the near IR regions [29–31]. Another interesting study is available in the
literature which focused on the measurements of several isotopologues of N 2 O i.e.
14 N 2
16 O,
14 N
15 N
16 O,
15 N
14 N
16 O and
14 N 2
18 O [32].
Recently, CRDS technique has been utilized to explore l-type doublings of OCS in
the mid-IR spectral region, where a widely tuneable mode-hop-free (MHF) externalcavity QCL was utilized as the light source [33]. Figure 8 demonstrates for this
type of measurements. Several rotationally resolved splitting of the l-type doublings
were recorded in the hot band transitions of OCS molecule, suggesting the enormous
capability of the CRDS technique for probing the ro-vibrational spectra with highresolution.
CRDS technique combined with an EC-QCL at 7.5 μm was also employed to
record the rotationally resolved several ro-vibrational lines in allowed (υ 4 + υ 5 )
0
and forbidden (υ 4 + υ 5 )
2 bands of C 2 H 2 [34] (Fig. 9).
The authors have determined the line intensities and air broadening coefficients
of the spectral transitions of C 2 H 2 . The values of the vibrational transition dipole
moment squared as well as empirical Herman-Wallis coefficients were estimated
from the experimental observations. All these spectroscopic data are very important
for astrophysical applications.
The group of Pradhan and co-workers has recently performed another interesting
high-resolution spectroscopic study for recording
13 C-isotopomer of methane (CH 4 )
by probing the ν 4 fundamental band at 7.5 μm [35]. They utilized the CRDS technique
297
5 Applications of Cavity Ring-Down Spectroscopy
CRDS is considered as one of the most popular techniques in analytical spectroscopy.
The enormous sensitivity that can be easily achieved in an optical cavity has allowed
us to detect the species in a variety of environments and consequently has found
applications in a wide range of fields. Here, we have mainly discussed some of its
important applications in trace gas sensing, high-resolution molecular spectroscopy
and medical diagnostics.
5.1 High-Resolution Molecular Spectroscopy
in the Gas-Phase by CRDS
Over the past decades, numerous studies have focused on the high-resolution spectroscopic investigation of several important molecular species and their isotopes such
as N 2 O, NO 2 , CH 4 , H 2 O, D 2 O, OCS etc. by CRDS technique. All these molecular
species have significant impact in environmental chemistry and biomedical science.
However, the Group of Romanini has utilized the CRDS technique for Dopplerbroadened measurements of N 2 O by probing the 6ν 3 − overtone band at 765 nm
[28].
There is another group, Quack and co-workers who have demonstrated the highresolution monitoring of several overtone and combination bands of CH 4 , chloroform
and N 2 O in the near IR regions [29–31]. Another interesting study is available in the
literature which focused on the measurements of several isotopologues of N 2 O i.e.
14 N 2
16 O,
14 N
15 N
16 O,
15 N
14 N
16 O and
14 N 2
18 O [32].
Recently, CRDS technique has been utilized to explore l-type doublings of OCS in
the mid-IR spectral region, where a widely tuneable mode-hop-free (MHF) externalcavity QCL was utilized as the light source [33]. Figure 8 demonstrates for this
type of measurements. Several rotationally resolved splitting of the l-type doublings
were recorded in the hot band transitions of OCS molecule, suggesting the enormous
capability of the CRDS technique for probing the ro-vibrational spectra with highresolution.
CRDS technique combined with an EC-QCL at 7.5 μm was also employed to
record the rotationally resolved several ro-vibrational lines in allowed (υ 4 + υ 5 )
0
and forbidden (υ 4 + υ 5 )
2 bands of C 2 H 2 [34] (Fig. 9).
The authors have determined the line intensities and air broadening coefficients
of the spectral transitions of C 2 H 2 . The values of the vibrational transition dipole
moment squared as well as empirical Herman-Wallis coefficients were estimated
from the experimental observations. All these spectroscopic data are very important
for astrophysical applications.
The group of Pradhan and co-workers has recently performed another interesting
high-resolution spectroscopic study for recording
13 C-isotopomer of methane (CH 4 )
by probing the ν 4 fundamental band at 7.5 μm [35]. They utilized the CRDS technique