Cavity-enhanced saturated absorption spectroscopy of the (30012) (00001) band
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Cavity-enhanced saturated absorption spectroscopy of the (30012) − (00001) band of 12C16O2 Cite as: J. Chem. Phys. 156, 044201 (2022); https://doi.org/10.1063/5.0074713 Submitted: 11 October 2021 • Accepted: 31 December 2021 • Accepted Manuscript Online: 03 January 2022 • Published Online: 24 January 2022 Y. Tan, Y.-R. Xu, T.-P. Hua, et al. ARTICLES YOU MAY BE INTERESTED IN Stronger orientation of state-selected OCS molecules with relative-delay-adjusted nanosecond two-color laser pulses The Journal of Chemical Physics 156, 041101 (2022); https://doi.org/10.1063/5.0075849 Two-color, intracavity pump–probe, cavity ringdown spectroscopy The Journal of Chemical Physics 155, 104201 (2021); https://doi.org/10.1063/5.0054792 Tip-enhanced Raman spectroscopy of confined carbon chains The Journal of Chemical Physics 156, 044203 (2022); https://doi.org/10.1063/5.0073950 J. Chem. Phys. 156, 044201 (2022); https://doi.org/10.1063/5.0074713 156, 044201 © 2022 Author(s).
The Journal
ARTICLE scitation.org/journal/jcp
of Chemical Physics
Cavity-enhanced saturated absorption
spectroscopy of the (30012) − (00001)
band of 12C16O2
Cite as: J. Chem. Phys. 156, 044201 (2022); doi: 10.1063/5.0074713
Submitted: 11 October 2021 • Accepted: 31 December 2021 •
Published Online: 24 January 2022
Y. Tan,1 , 2,a) Y.-R. Xu,1 T.-P. Hua,1 A.-W. Liu,1 , 2 J. Wang,1 , 2 Y. R. Sun,1 , 2 and S.-M. Hu1 , 2
AFFILIATIONS
1
Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, China
2
CAS Center for Excellence in Quantum Information and Quantum Physics, University of Science and Technology of China,
Hefei 230026, China
a)
Author to whom correspondence should be addressed: tanyan@ustc.edu.cn
ABSTRACT
The (30012) ← (00001) band of 12 C16 O2 in the 1.6 μm region is used for satellite observation of carbon dioxide in the Earth’s atmosphere.
Here, we report a Doppler-free spectroscopy study of this band with comb-locked wavelength-modulated cavity-enhanced absorption spec-
troscopy. Frequencies of 18 transitions with the rotational quantum numbers up to 42 were determined with sub-kHz accuracy, corresponding
to a fractional uncertainty at the 10−12 level. With this precision, we revealed an anomalous decrease of the line shift and an increase of the
line broadening for the Lamb dips of CO2 in the low-pressure regime compared to values obtained from Doppler-limited spectra at higher
pressures.
Published under an exclusive license by AIP Publishing. https://doi.org/10.1063/5.0074713
I. INTRODUCTION ← (00001) and (20012) ← (00001) bands.2,19–22 However, there are
still appreciable discrepancies in both transition frequencies and
Carbon dioxide (CO2 ) is the second strongest absorber in intensities for the (30012) ← (00001) band.12,23–27
infrared radiation after the water molecule. Therefore, CO2 is con- Recent progress in spectroscopy integrated with self-referenced
sidered the most important greenhouse gas, and it is the main optical frequency combs promotes the accuracy of molecular spec-
target for numerous remote sensing missions, such as the NASA troscopy to a new stage at the kHz level. The improvements could
Orbiting Carbon Observatory (OCO-2/3),1,2 the greenhouse gases be useful to refine the energy levels of molecules, test theoretical
observing satellite (GOSAT),3 the Chinese Carbon Dioxide Obser- results based on empirical models,22 and ab initio calculations.28
vation Satellite Mission (TanSat),4 and the ground-based Total Cavity ring-down spectroscopy (CRDS) of Doppler-broadened CO2
Carbon Column Observing Network (TCCON).5,6 In these satellite- lines near 1.6 μm has been reported by several groups.23,26,29–33 Long
based missions, CO2 infrared absorption bands at 1.6 and 2.06 μm et al.25 carried out cavity ring-down spectroscopy (CRDS) mea-
are extensively used, where the strong absorption is mainly con- surements of Doppler-broadened spectra of the (30012) ← (00001)
tributed from the (30012), (30013), and (20012) bands. CO2 data band and reported line centers with combined uncertainties of a few
retrieval algorithms require accurate spectroscopic parameters,7,8 tens of kHz. Later, Gotti et al.34 reported the P(12)e transition in the
including line positions better than 0.0001 cm−1 , line intensities same band with a claimed uncertainty of 2.1 kHz. Recently, Reed
at sub-percent accuracy, and more complicated line-shape mod- et al.31 presented line centers in the same band with an accuracy
els beyond the Voigt profile.9,10 Theoretical calculations of the as high as 0.2 kHz. They also reported saturation spectroscopy of
line centers and intensities of CO2 have been extensively imple- P(10)e and R(16)e lines in this band with uncertainties of a few kHz
mented by first-principles or effective Hamiltonian and dipole using the CRDS measurements. Although the Doppler-broadening
moment models.11–18 Excellent agreement between calculations and width of CO2 at room temperature is about 350 MHz around
experimental results has been achieved for both the (30013) 1.6 μm, line centers with kHz uncertainty have been derived from
J. Chem. Phys. 156, 044201 (2022); doi: 10.1063/5.0074713 156, 044201-1
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of Chemical Physics
fitting Doppler-broadened spectra with sophisticated line profile
models. However, there have been reports on the nonlinear depen-
dence of the line shift and broadening of molecules at low pressures
in the early works of saturation spectroscopy.35–37 Therefore, it is
worth checking the consistency between the line parameters derived
from Doppler-broadened spectra and those from high-precision
Doppler-free measurements. We have applied the comb-locked
cavity ring-down saturation spectroscopy in Lamb-dip measure-
ments of molecules in the near-infrared region,28,38,39 including the
(30013) ← (00001) band of CO2 .22 In these Doppler-free spec-
troscopy measurements, widths of observed Lamb dips have been
reduced to a few hundred kHz. Consequently, line overlapping was
eliminated and line centers could be determined with uncertainties
of a few kHz or even better.38 In addition, we have observed larger FIG. 1. The experimental setup of wavelength-modulated cavity-enhanced absorp-
tion spectroscopy (WM-CEAS). ECDL: external-cavity diode laser; AOM: acousto-
line broadening coefficients compared to the Doppler-broadened optical modulator; EOM: electro-optical modulator; TEC: temperature controller;
results for both CO and CO2 , but we are not able to see any line shift PD: photodiode detector; PI: proportion integration amplifier; PZT: piezoelectric
in the pressure region below 1.5 Pa. Meanwhile, concerns have been actuator; PC: personal computer.
raised40,41 that the change of laser power in a ring-down event might
subsequently affect the CRDS signal in the saturation regime, which
may lead to a frequency shift and broadening in observed saturation
absorption profiles. cavity was used to lock the laser frequency to one longitudinal mode
Herein, we present saturation spectroscopy of 12 C16 O2 of the cavity via the PDH scheme. Since the sensitivity of CEAS is
using the wavelength-modulated cavity-enhanced absorption spec- sensitive to the intra-cavity laser power, a power stabilization servo
troscopy (WM-CEAS) method. Intra-cavity power intensities were is applied through the feedback signal on the AOM and the rela-
enhanced up to 10 kW/cm2 when an mw-power probe laser was tive deviation of the probe laser power before entering the cavity
tightly locked with a high-finesse cavity. Meanwhile, the input is within 0.1%/min. In the meantime, the detector PD3 monitors
laser power was stabilized through a feedback servo. The method the beat signal between the laser and an Optical Frequency Comb
allows us to measure Lamb-dip spectra of molecules with sensi- (OFC), which serves as the absolute frequency reference. The OFC
tivity comparable to the best CRDS measurements and to mea- is synthesized by an Er:fiber oscillator with its repetition frequency
sure the line profile more accurately without the influence due ( fr ≈ 184 MHz) and carrier offset frequency ( f0 ≈ 20 MHz) refer-
to changing laser power in CRDS. Center frequencies of 18 tran- enced to a local active hydrogen maser (VCH-1003M). A locking
sitions from P(42)e to R(26)e in the (30012) ← (00001) band circuit is applied to stabilize the beat frequency by acting on the cav-
were determined with uncertainties below 1 kHz. Comparisons ity length through a piezoelectric actuator (PZT) attached to one of
to previous studies are presented and discrepancies between the the HR mirrors. Therefore, the absolute frequency of our probe laser
results from this work and those from Doppler-limited measure- can be determined by
ments are discussed as well. The power dependence and pressure
dependence of the centers and widths of the Lamb dips are also ν = f0 + N fr + fA + fB, (1)
investigated.
where f0 and fr are the carrier offset frequency and the repetition
II. EXPERIMENTS frequency of the OFC, and fA and fB are the radio frequency driv-
ing the AOM and the beat frequency between the laser and the OFC,
Configuration of the comb-locked cavity-enhanced absorption respectively. Direct CEAS signal (PD4) could be acquired by tun-
spectroscopy setup is shown in Fig. 1, which is similar to that used ing the beat frequency, and the WM-CEAS (1 f ) signal is obtained
in our recent studies.42,43 This method relies on a high-finesse cavity by a lock-in amplifier (SRS 830) when we modulate the laser wave-
to enhance both the effective absorption path length and the intra- length through a dither signal applied to the PZT. A carbon dioxide
cavity laser power by a factor of 103 –105 , related to the reflectivity gas sample with 99.99% purity was used in this experiment, and the
of the mirrors. An external cavity diode laser (ECDL, Toptica DL temperature of the sample cavity was kept around 295 K with a drift
Pro) operating at 1.57 μm was first frequency-shifted by an acousto- less than 0.2 K/day.
optical modulator (AOM), phase modulated by an electro-optical
modulator (EOM), and then introduced into the high-finesse cavity.
The EOM was temperature-stabilized by a temperature controller III. RESULTS AND DISCUSSION
(TEC) unit to reduce the residual amplitude modulation (RAM) Representative CEAS and WM-CEAS (1 f ) spectra obtained
effect. The optical cavity is composed of two high-reflective (HR) from a single scan in 30 s are given in Fig. 2. The CEAS spec-
mirrors (R = 99.997%) with a distance of 44.6 cm, corresponding trum shown in Fig. 2(a) was fitted by a Lorentzian function, and
to a free spectral range (FSR) of 336 MHz. There were two detec- fitting residuals are given in the lower panel, indicating a signal-
tors (PD1 and PD2 see in Fig. 1) used before the sample cell. PD1 to-noise ratio (SNR) of 100. Drift in the baseline was effectively
monitoring the input laser power was applied to stable the input reduced by the frequency demodulation, and the WM-CEAS (1 f )
laser. In addition, PD2 detecting the back-reflected signal from the spectrum could be well reproduced by the first derivation (1 f ) of the
J. Chem. Phys. 156, 044201 (2022); doi: 10.1063/5.0074713 156, 044201-2
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of Chemical Physics
FIG. 2. A single scan of the saturation spectrum of the R(16)e line in the (30012) ← (00001) band recorded by (a) direct CEAS and (b) wavelength-modulated (wm) CEAS
(1 f ) at a pressure of 1 Pa. Lorentz profile and the first derivative of Lorentz profile were applied to fit two spectra, respectively. Fitting residuals shown in (c) and (d) were
multiplied by a factor of 100.
Lorentzian function, 0.1–0.4. The results for the line centers, as shown in the upper panel
of Fig. 3(a), indicate that no power dependence of the line center is
2A τ Γ
1f found in this work. However, a linear dependence of the linewidth
SL (ν) = {
πτ ∫0 [Δν + νa cos(2π f m t)]2 + Γ2
cos(2π f m t)dt},
on the input laser power was observed, as plotted in Fig. 4(a).
(2) Sample pressures ranging from 0.5 to 4.0 Pa were used to inves-
where Δν = ν − ν0 corresponds to the detuning frequency from tigate the collision-induced effects; hence, the P(10)e line centers and
the line center ν0 . Parameters for the line center ν0 , the half- widths obtained from fitting the spectra with 1 f − Lorentzian profiles
width at half maximum (HWHM) Γ, the amplitude of the Lamb [also see in Eq. (2)] are given in Figs. 3(b) and 4(b). The estimated
dip A, together with the modulation amplitude νa were retrieved saturation parameter S decreased from 0.7 to 0.03 when the sample
from the fitting procedure. The modulation frequency fm and the pressure increased from 0.5 to 4.0 Pa at the same input laser power
lock-in integral time τ were fixed as 250 Hz and 30 ms, respec- of 4 mW. We constrained our measurements with pressures lower
tively. The SNR of the WM-CEAS spectrum was improved by a than 1 Pa for the other transitions except for the P(10) transition as
factor of 6 compared to CEAS, as shown in Fig. 2. In order to discussed before, because the amplitude of the Lamb dip decreases
find any systematic shift in the measurements, we repeated the at higher pressures, and the uncertainty due to the collision-induced
frequency measurements for the R(9) and R(10) (3-0) transitions shift increases with pressure. As shown in Fig. 3(b), we did see a
of 12 C16 O. Line positions derived from the recorded WM-CEAS pressure dependence of the line center about −0.35(±0.08) kHz/Pa,
spectra are 191 360 212 763.5(7) and 191 440 612 665.2(5) kHz for but it is about 1/5 of the normal pressure shift coefficient
the R(9) and R(10) lines, respectively. Deviations from the previ- determined from the Doppler-limited measurements,25 which is
ously reported results obtained by comb-locked cavity ring-down −1.7 kHz/Pa for the P(10)e transition. The anomalous decrease of
spectroscopy38,39 are 0.5 and 0.1 kHz, which are within the com- the line shift of the Lamb-dip center in the low-pressure regime was
bined uncertainty of these two measurements, indicating good first reported in the early work by Bagaev et al. in the case of helium-
repeatability. and xenon-broadened methane spectra.35 Our results further con-
We measured the Lamb dips of the P(10)e transition with dif- firmed that the decrease of the pressure-induced line shift of the
ferent input laser powers to demonstrate the powerful influence on Lamb dip at low pressures could practically reduce the uncertainty
the transition frequency, as shown in Figs. 3(a) and 4(a). The satura- from the pressure shift, which could be an advantage for frequency
tion power is estimated44 to be about 89 W for the P(10) transition standards based on molecular transitions. We also found that the
at the pressure of 1 Pa with a beam waist radius of about 0.07 cm in width of the Lamb dip increases quite linearly with both the input
the cavity when we considered the Doppler-broadened absorption laser power and the sample pressure, as shown in Fig. 4. Moreover,
simulated from the HITRAN2020.45 In our measurements, we sta- the determined self-pressure broadening coefficient of 105.5(±4.8)
bilized the probe laser power before entering the cavity, which was kHz/Pa is much larger than the normal broadening coefficient of
in the range of 1–4 mW, corresponding to a saturation parameter of 47.5 kHz/Pa obtained from the Doppler-limited measurements.25
J. Chem. Phys. 156, 044201 (2022); doi: 10.1063/5.0074713 156, 044201-3
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of Chemical Physics
FIG. 3. Line position of the P(10)e line in the (30012) ← (00001) band measured by WM-CEAS. (a) Line positions measured at different input laser powers (1–4 mW)
with the same sample pressure 1 Pa. The red line and band indicate the linear fitting result with 1σ confidence. (b) Line positions measured at different sample pressures
(0.5–4 Pa) with the input laser power of 4 mW. The blue line and band indicate the linear fitting analysis with 1σ confidence. The dashed line indicates the line shift coefficient
obtained from Doppler-broadened spectroscopy of CO2 at higher pressures.25 The error bars are corresponding to the standard deviation of the data obtained under the
same condition. (c) Line positions obtained from each WM-CEAS scan (740 in total). (d) Histogram plot of the line frequencies in 1 kHz bins with overlaid normal distribution.
Inset: Allan deviation of the line position.
Moreover, the self-pressure broadening coefficient is corrected to were collected in Fig. 3(c). The averaged line center for the P(10)e
140.6(±5.5) kHz/Pa if we take into account the change of saturation transition is 190 059 681 727.00 kHz with a statistical uncertainty of
parameter S due to the pressure broadening effect. In summary, the 0.25 kHz. The Allan deviation of the line centers vs the number of
results show that both the line broadening and shift parameters in scans is shown as the inset in Fig. 3(d), indicating that there is no
the low-pressure region (P < 10 Pa) were quite different from those systematic drift in the measurements. Hence, we gave a statistical
in the normal pressures, which could also be rather useful in study- uncertainty of 0.25 kHz for the averaged line center after averaging
ing the mechanism of molecular interactions involving both elastic about 100 scans.
and inelastic scattering.35,36 An uncertainty budget of the P(10)e line position could be
We recorded the Lamb-dip spectra of the P(10)e line with alto- found in Table I. The statistical uncertainty of 0.25 kHz was derived
gether about 740 WM-CEAS scans in ∼7 h. The line centers derived according to the results shown in Fig. 3. The carrier offset frequency
from all the scans with different input laser powers and sample ( f0 ) as well as the beating signals ( fB ) between the probe laser and
pressures corrected with the corresponding pressure shift coefficient the OFC were recorded by two frequency counters. We corrected
J. Chem. Phys. 156, 044201 (2022); doi: 10.1063/5.0074713 156, 044201-4
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FIG. 4. Line widths [full width at half maximum (FWHM)] of the P(10)e line in the (30012) ← (00001) band measured at different input laser powers and sample pressures.
(a) Different laser powers, with the same sample pressure of 1 Pa. (b) Different sample pressures, with the same input laser power of 4 mW. The dashed line indicates the
collision-induced broadening coefficient obtained from Doppler-broadened spectroscopy of CO2 at higher pressures.25 The error bars are corresponding to the standard
deviation of the data obtained under the same condition.
the carrier offset frequency ( f0 ) to the averaging results during each temperature of 295 K, we estimate a frequency shift of −0.18(1) kHz
experiment and the maximum correction was about 100 Hz. Since due to the second-order Doppler effect. The AC Stark shift caused
the OFC was referenced with the hydrogen maser, we give accu- by the intra-cavity laser power and DC Stark shift from the static
racy better than 1 × 10−12 (0.19 kHz at 1.57 μm), considering the molecular polarizability were estimated to be within a few tens of
attenuation effects in the OFC locking system. The radio frequency hertz, which were neglected in our measurements. Besides, there is
driving AOM has a fluctuation below 50 Hz mainly from the drift typical outgassing of water vapor from the sample cell, which is less
of the room temperature. The results shown in Figs. 3(a) and 3(b) than 0.5 Pa/day as monitored for days. The required time for each
indicate that no evidence of power shift and the pressure-induced experiment at a certain pressure is about 1 h, leading to a line shift of
shift is within our experimental uncertainty when the sample pres- about 40 Hz. Note that we assumed the pressure-induced line shift
sure is lower than 1 Pa, and we gave each an uncertainty of 0.25 in the Doppler-broadened spectra, which may be overestimated in
and 0.35 kHz as discussed in Sec. II. We included uncertainty of our case. The uncertainty of about 40 Hz was included in the final
0.2 kHz contributed from the line profile although we did not see budget of line position accuracy. Hence, the comprehensive uncer-
evidence of asymmetry from the fitting residuals. Considering a tainty is 0.62 kHz and the P(10)e line frequency is determined to be
root-square mean velocity of 409 m/s of CO2 molecules at the room 190 059 681 727.00(62) kHz.
We have recorded Lamb dips of 18 transitions in the (30012)
← (00001) band. Line positions together with the uncertainties
can be found in Table II. Two pairs of lines with the same upper
TABLE I. The uncertainty budget for the position of the P(10)e line in the (30012) level, R(12)e and P(14)e, and R(16)e and P(18)e, were selected to
← (00001) band of 12 C16 O2 (unit: kHz). check the consistency of the experimental accuracy. The ground-
state combination difference of each pair of transitions was derived.
Source Frequency Uncertainty
We got 631 638 352.0(1.1) kHz for the energy difference between
Statistical 190 059 681 727.00 0.25 rotational levels J ′′ = 14 and J ′′ = 12 and 818 721 045.1(0.9) kHz
Comb frequency −0.05 0.19 for that between J ′′ = 18 and J ′′ = 16. They agree well with those
Locking servo 0.18 given in our previous study of the (30013) ← (00001) band,22
AOM frequency −0.12 0.05 which were 631 638 353.8(1.1) and 818 721 047.0(1.2) kHz, respec-
Pressure shift 0.35 tively. Deviations are 1.8 and 1.9 kHz, within 2σ of the combined
Power shift 0.25 uncertainties.
Line profile asymmetry 0.20 The transition frequencies determined in this work were also
Second-order Doppler +0.18 0.01 compared to those values given in the databases of HITRAN2016,46
Shift due to outgassing 0.04 CDSD-296,16 and HITRAN202024,45 as well. Figure 5 shows an
Total 190 059 681 727.00 0.62 agreement within 0.5 MHz for J lower than 40 for all of them, but
the residuals for CDSD-296 and HITRAN2020 are slightly smaller
J. Chem. Phys. 156, 044201 (2022); doi: 10.1063/5.0074713 156, 044201-5
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TABLE II. Frequencies of the lines in the (30012) ← (00001) band of 12 C16 O2 , along with deviations between this work and previous studies as well as the HITRAN2020
database. Values in brackets correspond to 1σ uncertainties. All values are in the unit of kHz.
Transitions This work Δν, Guo et al.26 Δν, Reed et al.32 Δν, Long et al.25 Δν, HITRAN24,45
P(42)e 189 131 192 886.4(0.6) 34(120) 46.7(9.4) 274(809) 813
P(40)e 189 195 523 790.8(0.6) −121(110) −12.3(0.8) 243(570) 573
P(36)e 189 321 765 443.2(0.7) −233(90) −3.8(0.8) −304(120) 326
P(32)e 189 444 719 646.3(0.6) 64(110) 1.2(0.6) 94(180) 3
P(30)e 189 504 943 247.7(0.8) −338(100) −7.5(0.8) −90(90) −90
P(24)e 189 680 512 798.7(0.6) −189(110) 1.4(0.5) −86(120) −85
P(18)e 189 848 300 361.2(0.6) 19(120) 1.2(1.4) −44(60) −105
P(14)e 189 955 768 018.8(0.7) −49(100) −5.4(0.5) −51(60) 38
P(12)e 190 008 170 200.8(0.6) −1(100) −1.4(0.5) −100(30) 80
P(10)e 190 059 681 727.0(0.6) 123(100) −2.5(0.3) −57(60) 93
P(6)e 190 160 019 648.5(0.9) 12(110) −4.1(2.9) −171(90) 9
P(2)e 190 256 762 624.4(0.9) 286(80) −1.0(0.8) −150(180) 59
R(2)e 190 372 618 521.7(0.5) 4.5(0.7) 157(270) 157
R(6)e 190 461 239 481.8(0.9) −8.6(2.5) 186(60) 246
R(12)e 190 587 406 370.8(0.8) 2.1(1.2) −67(60) 113
R(16)e 190 667 021 406.3(0.7) 3.4(0.2) 1(30) 61
R(20)e 190 743 064 155.5(0.7) −31(60) −31
R(26)e 190 850 487 005.2(0.8) 342(60) −48
P(14)e Δν, Gatti et al.29 Δν, Truong et al.33
3.2(3.0) 4.2(9.0)
P(12)e Δν, Gotti et al.34
−20.3(2.1)
Δν, Reed et al.31 ,a
P(10)e −3.9(1.8)
R(16)e 5.0(1.9)
a
Results were taken from saturation spectroscopy using CRDS measurement,31 while other literature frequencies were from Doppler-broadened CRDS measurements. Besides, Δν,
HITRAN are taking from the recent updated HITRAN2020.24,45
than those for HITRAN2016, and they are all getting worse when
outside this range. Moreover, a clear J-dependence is observed from
the deviations, indicating that the effective Hamiltonian for this state
needs to be refined.
Several high-accuracy line positions in this band have been
reported in recent years.25,26,29,32–34 A comparison between the line
positions obtained in this work and previous experimental results
is plotted in Fig. 6 and listed in Table II as well. In general, previ-
ous results obtained from Doppler-limited measurements25,26,29,32–34
agree well with this work, the corresponding deviations, and their
uncertainties varying from several kHz to a few hundred kHz, as
illustrated in Fig. 6(a). Deviations are from 1 to 46 kHz between
our transition frequencies and those given in a very recent Doppler-
limited study by Reed et al.32 Note that for quite a few transitions,
the discrepancies are significant: up to 10σ, as illustrated in Fig. 6(b).
Meanwhile, the P(10)e and R(16)e line centers given from Lamb-dip
measurements by the same group31 agree with our results, and the
deviations are about 2σ. It is also worth noting that the P(12)e transi-
tion frequency derived from Doppler-limited CRDS measurements
FIG. 5. Differences between the line positions obtained in this work and those by Gotti et al.34 has a 20 kHz deviation (9σ from our result). We
given in the HITRAN2016 (squares),46 the CDSD-296 (circles),16 and the speculate that the anomalous decrease of the collision-induced shift
HITRAN2020(stars slightly different from the CDSD-296)24,45 databases. Here, of the Lamb dips in the low-pressure region may explain the dis-
m = −J′′ and J′′ + 1 for the P and R branch lines.
crepancies between our line centers and those from Doppler-limited
J. Chem. Phys. 156, 044201 (2022); doi: 10.1063/5.0074713 156, 044201-6
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of Chemical Physics
FIG. 6. (a) The differences between the line positions obtained in this work and those literature values for the (30012) ← (00001) band. The referenced experimental
data using Doppler-limited measurements were taken from Reed et al.,31,32 Guo et al.,26 Gotti et al.,34 Long et al.,25 Gatti et al.,29 and Truong et al.33 Here, m = −J′′ and
J′′ + 1 for the P and R branch lines, respectively. Note that red squares (Reed et al.) represent the CRDS saturated spectroscopy results from Reed et al.31 (b) A zoomed
comparison of the differences shown in the left panel.
studies since the self-induced line shift from Doppler contour was determined transition frequencies could be used in future refine-
used to correct the line frequency at zero-pressure. It demands fur- ments of the effective Hamiltonian.
ther investigation of the collision effects in the low-pressure region.
Note that the line intensity and profile parameters, together with the
ACKNOWLEDGMENTS
line center, were derived simultaneously from fitting the Doppler-
broadened spectra, and deviations in the line center may also cause This work was jointly supported by the National Natural
variance in other parameters obtained in the fitting, particularly for Science Foundation of China (Grant Nos. 41905018, 21903080,
the line shift coefficients due to strong correlation between them. and 21688102) and the Chinese Academy of Sciences (Grant No.
Therefore, we believe that our results could be used in the analysis XDC07010000).
of the Doppler-limited spectra to retrieve more accurate line shape
parameters. AUTHOR DECLARATIONS
Conflict of Interest
IV. CONCLUSION
The authors declare that they have no conflicts of interest to
To summarize, we have performed cavity-enhanced satu- this work.
rated absorption spectroscopy of the (30012) ← (00001) band for
12 16
C O2 . In the measurements, both the probe laser and the high- DATA AVAILABILITY
finesse cavity were locked with an optical frequency comb and
fractional frequency accuracy as good as 1 × 10−12 was achieved. We The data that support the findings of this study are available
revealed an unusual sharp decrease of the line shift and increase of from the corresponding author upon reasonable request.
the line broadening of the Lamb dips at pressures less than 4 Pa,
which indicate a need for further investigations in the molecular col- REFERENCES
lisions and interactions at the low-pressure region. Frequencies of 18 1
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