Millimeter-wave-detected, millimeter-wave optical polarization spectroscopy

Article abstract of DOI:10.1063/1.2069865

We report a new form of microwave optical double-resonance spectroscopy called millimeter-wave-detected, millimeter-wave optical polarization spectroscopy (mmOPS). In contrast to other forms of polarization spectroscopy, in which the polarization rotation of optical beams is detected, the mmOPS technique is based on the polarization rotation of millimeter waves induced by the anisotropy from optical pumping out of the lower or upper levels of the millimeter wave transition. By monitoring ground-state rotational transitions with the millimeter waves, the mmOPS technique is capable of identifying weak or otherwise difficult-to-observe optical transitions in complex chemical environments, where multiple molecular species or vibrational states can lead to spectral congestion. Once a transition is identified, mmOPS can then be used to record pure rotational transitions in vibrationally and electronically excited states, with the resolution limited only by the radiative decay rate. Here, the sensitivity of this nearly-background-free technique is demonstrated by optically pumping the weak, nominally spin-forbidden CS e Σ-3 -X Σ+1 (2-0) and d Δ3 -X Σ+1 (6-0) electronic transitions while probing the CS X Σ+1 (v″ =0, J″ =2-1) rotational transition with millimeter waves. The J′ =2, N′ =2← J′ =1, N′ =1 pure rotational transition of the CS e Σ-3 (v′ =2) state is then recorded by optically preparing the J′ =1, N′ =1 level of the e Σ-3 (v′ =2) state via the J′ =1, N′ =1← J″ =1 transition of the e Σ-3 -X Σ+1 (2-0) band.

Full text of DOI:10.1063/1.2069865

Millimeter-wave-detected, millimeter-wave optical polarization spectroscopy
Adam H. Steeves, Hans A. Bechtel, Stephen L. Coy, and Robert W. Field
Citation: The Journal of Chemical Physics 123, 141102 (2005); doi: 10.1063/1.2069865
View online: http://dx.doi.org/10.1063/1.2069865
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THE JOURNAL OF CHEMICAL PHYSICS 123, 141102 ͑2005͒
Millimeter-wave-detected, millimeter-wave optical polarization spectroscopy
Adam H. Steeves, Hans A. Bechtel, Stephen L. Coy, and Robert W. Fielda͒
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
͑Received 9 August 2005; accepted 24 August 2005; published online 12 October 2005͒
We report
a new form of microwave optical double-resonance spectroscopy called
millimeter-wave-detected, millimeter-wave optical polarization spectroscopy ͑mmOPS͒. In contrast
to other forms of polarization spectroscopy, in which the polarization rotation of optical beams is
detected, the mmOPS technique is based on the polarization rotation of millimeter waves induced
by the anisotropy from optical pumping out of the lower or upper levels of the millimeter wave
transition. By monitoring ground-state rotational transitions with the millimeter waves, the mmOPS
technique is capable of identifying weak or otherwise difficult-to-observe optical transitions in
complex chemical environments, where multiple molecular species or vibrational states can lead to
spectral congestion. Once a transition is identified, mmOPS can then be used to record pure
rotational transitions in vibrationally and electronically excited states, with the resolution limited
only by the radiative decay rate. Here, the sensitivity of this nearly-background-free technique is
demonstrated by optically pumping the weak, nominally spin-forbidden CS e 3⌺−−X 1⌺+ ͑2-0͒ and
d 3⌬−X 1⌺+ ͑6-0͒ electronic transitions while probing the CS X 1⌺+ ͑ =0,J =2-1͒ rotational
v
Љ
Љ
transition with millimeter waves. The J =2,N =2←J =1,N =1 pure rotational transition of the
Ј
Ј
Ј
Ј
CS e 3⌺− ͑ =2͒ state is then recorded by optically preparing the J =1,N =1 level of the
v
Ј
Ј
Ј
e 3⌺− ͑ =2͒ state via the J =1,N =1←J =1 transition of the e 3⌺−−X 1⌺+ ͑2-0͒ band. © 2005
v
Ј
Ј
Ј
Љ
American Institute of Physics. ͓DOI: 10.1063/1.2069865͔
Because the frequencies of pure rotational transitions are
easily calculated with high precision, the Ͻ1 MHz resolution
of microwave sources makes rotational transitions straight-
forward to resolve and assign even in complex chemical en-
vironments, such as in discharges, where spectral congestion
can hinder assignments of optical transitions. As a conse-
quence, microwave transitions have been measured and
tabulated1,2 for a vast number of transient species, many of
which have unknown optical transitions. Microwave-
detected, microwave-optical double-resonance ͑MODR͒
techniques,3–7 which monitor resonant changes in the micro-
wave signal induced by optical pumping out of the lower or
upper state of the rotational transition, are a convenient
method of identifying optical transitions in these molecules.
By labeling the molecule and the lower rotational level of the
optical transition, microwave-detected MODR schemes pro-
vide a “rotational handle” to tremendously simplify the spec-
trum, allowing easier assignments and species identification.
Moreover, microwave-detected MODR techniques are
widely applicable; they can be applied to weak vibrational
and electronic transitions and to transitions that have poor
fluorescence quantum yields, which arise from nonradiative
processes, such as collisional quenching, predissociation, in-
tersystem crossing, or internal conversion. Once optical tran-
sitions are identified, microwave-detected MODR can then
be used to measure the pure rotational spectra of optically
populated vibrationally and electronically excited molecules,
with the resolution limited only by the radiative decay rate.
Then, by monitoring the excited rotational transition, addi-
tional optical transitions from this state can be identified.
Thus, microwave-detected MODR not only provides a rota-
tional handle for identifying optical transitions, but it can
also be used to map out the energy-level structure of the
molecule by systematically bootstrapping through different
vibrational and electronic transitions, with each vibronic
level unambiguously labeled by its characteristic pure rota-
tional transition.
Here, we report a new microwave-detected MODR tech-
nique applicable to the millimeter-wave frequency region
called millimeter-detected, millimeter-wave optical polariza-
tion spectroscopy ͑mmOPS͒. mmOPS is analogous to optical
polarization spectroscopy,8 polarization labeling,9 and
microwave-optical polarization spectroscopy ͑MOPS͒,10–12
in which the polarization of a probe beam is rotated by the
angular anisotropy ͑unequal population in MJ sublevels͒ cre-
ated by a polarized pump beam. Unlike previously reported
polarization-detected techniques, however, in the current
implementation of mmOPS, the polarization rotation is in the
millimeter-wave beam, rather than the polarization rotation
of an optical beam. We apply mmOPS to the well-known
A 1⌸−X 1⌺+ electronic band system13 of CS, which is pro-
duced in a pulsed discharge supersonic nozzle. We find that
the near-zero background of mmOPS makes it more sensitive
than the millimeter-wave-detected, millimeter-wave optical
double-resonance ͑mmODR͒ technique,5 especially when us-
ing a liquid-helium-cooled InSb detector.
Figure 1 is a schematic of the mmOPS apparatus. The
experiment is performed in a 12ϫ12ϫ12 in.3 vacuum
chamber, which is evacuated by a 6 in. diffusion pump ͑Diff-
a͒
Author to whom correspondence should be addressed. Electronic mail:
rwfield@mit.edu
0021-9606/2005/123͑14͒/141102/4/$22.50
123, 141102-1
© 2005 American Institute of Physics
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141102-2
Steeves et al.
J. Chem. Phys. 123, 141102 ͑2005͒
FIG. 1. Schematic of mmOPS experimental apparatus. The millimeter-wave source is a Gunn oscillator ͑73–106 GHz͒, stabilized and scanned by a
phase-locked loop ͑PLL͒. The millimeter-wave radiation is collimated and focused by a pair of PFTE lenses ͑f =40 cm and f =30 cm, respectively͒ into a
supersonic molecular beam produced from a pulsed dc discharge nozzle. After exiting the chamber, the millimeter-waves are recollimated and focused with
another set of PFTE lenses ͑f =30 cm and f =40 cm, respectively͒ onto a liquid-helium-cooled InSb detector. The polarization rotation of the millimeter-waves
is detected with nominally crossed freestanding wire grid polarizers.
stak 160, Edwards͒ backed by a rotary mechanical pump. CS
is generated in a supersonic expansion by passing a mixture
of 1% CS2 in Ar through a pulsed discharge nozzle
͑1-mm-diameter orifice͒ that is similar to the design of Sanz
et al.14 The optimal experimental conditions for detection of
millimeter-wave radiation is optimized by expanding the op-
tical beam to ϳ2 cm in diameter in the overlap region.
Figure 2 shows selected rotational transitions from the
CS A 1⌸−X 1⌺+ ͑1-0͒ vibrational band, obtained using the
millimeter-detected mmODR ͓Fig. 2͑a͔͒ and mmOPS ͓Fig.
2͑b͔͒ techniques. In the mmODR technique, the millimeter-
wave frequency is locked onto the CS X 1⌺+ ͑ =0,J =2
−1͒ rotational transition, and the absorption of the
millimeter-wave radiation is monitored while the laser fre-
quency is scanned. When the lower state of the optical tran-
sition is one of the two levels connected by the millimeter-
wave transition, the sudden change in the ground-state
population induced by the laser pulse creates a millimeter-
wave transient nutation signal.3–5 If the optical transition is
out of the upper millimeter-wave connected level, then the
transient nutation signal is absorptive ͑positive͒, whereas if
the optical transition is out of the lower state, the transient
nutation signal is emissive ͑negative͒. In addition to the five
the CS X 1⌺+͑ =0,J=2−1͒ rotational transition are backing
v
pressure ͑3 atm͒, nozzle pulse duration ͑350 ␮s͒, negative
discharge voltage ͑1.4 kV͒, and discharge pulse ͑1 ms dura-
tion͒.
v
Љ
Љ
The millimeter-wave radiation is produced by a W-band
͑75–100 GHz͒ Gunn oscillator ͑J. E. Carlstrom Co.͒ that is
phase-locked ͑XL Microwave 800A͒ to a harmonic of a mi-
crowave synthesizer ͑HP 8672A͒ and coupled through wave-
guide components to a calibrated attenuator ͑Hitachi
W9513͒. The radiation is emitted into free space through a
standard gain horn ͑TRG Control Data, 15 dB͒ with a linear
polarization of 45° relative to the pump-laser polarization
and focused by a pair of polytetrafluoroethylene ͑PTFE͒
lenses ͑f =40 cm, f =30 cm͒ to a spot size of ϳ1 cm diam-
eter at the point of intersection with the molecular beam
ϳ2 cm downstream from the nozzle. After exiting the cham-
ber, the millimeter-wave beam is refocused by a second pair
of PTFE lenses ͑f =30 cm, f =40 cm͒ onto a liquid-helium-
cooled InSb hot-electron bolometer ͑Cochise Instruments͒.
The bolometer output is digitized with a 500 MHz oscillo-
scope ͑LeCroy LC334A͒ and transferred to a computer for
storage. The rotation of the millimeter-wave polarization is
detected by a pair of nearly crossed freestanding wire grid
polarizers. The polarizing grids are composed of
25-␮m-diameter gold-plated tungsten wire with center-to-
center wire spacings of 100 ␮m.15 When perfectly crossed,
the polarizers have an extinction ratio of ϳ10−3.
expected optical transitions originating from
J
Љ
The ultraviolet radiation is produced by frequency dou-
bling the output of a Nd3+:YAG-pumped dye laser ͑Quanta
Ray DCR-3/Lambda Physik 3002͒ in a beta-barium borate
͑BBO͒ crystal. The doubled radiation ͑ϳ1–1.5 mJ/pulse͒ is
scanned ϳ10 cm−1 by pressure tuning ͑N2͒ the etalon-
narrowed oscillator cavity and is calibrated to 0.01 cm−1
using a small amount of the residual fundamental to record
an I2 fluorescence spectrum.16 As shown in Fig. 1, the verti-
cally polarized ultraviolet radiation enters the vacuum cham-
ber in a direction orthogonal to both millimeter-wave and
molecular beams. The overlap of the optical field with the
FIG. 2. Comparison of ͑a͒ the millimeter-wave-detected mmODR technique
with ͑b͒ the millimeter-wave-detected mmOPS technique. The spectra are
obtained by scanning the laser over the A 1⌸−X 1⌺+ ͑1-0͒ CS band, while
the millimeter-wave frequency is locked to the CS X 1⌺+
͑
=0,J =2-1͒
v
Љ Љ
rotational transition. The laser was scanned at the same rate for both spectra
and each point is an average of 20 laser shots. The expanded baselines in the
region of 39 829 cm−1 indicate that the baseline noise is improved by a
factor of 4 for mmOPS relative to mmODR.
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141102-3
Millimeter-wave optical polarization spectroscopy
J. Chem. Phys. 123, 141102 ͑2005͒
=1͓Q͑1͒,R͑1͔͒ and J =2͓P͑2͒,Q͑2͒,R͑2͔͒, Fig. 2͑a͒ shows
Љ
a negative-going transition at the expected position of the
R͑0͒ line. The sign of the extra signal indicates that the op-
tical transition out of J =0 causes a preferential depopula-
Љ
tion of the J =1 level relative to the J =2 level, which is
Љ
Љ
most likely due to collisional ͑J =0 hole filling from J =1͒
Љ
Љ
population transfer following optical excitation. The optical
pump can significantly alter the thermal population differ-
ence between the J =2 and J =1 levels because the CS
Љ
Љ
A 1⌸−X 1⌺+ ͑1-0͒ transition is fully allowed and the Franck-
Condon factor is 0.14.13 As a consequence, both negative and
positive mmODR signals are observed that are considerably
larger than the original absorption signal in the absence of
optical pumping.
The sensitivity of the mmODR experiments is limited
primarily by the large and fluctuating CS absorption back-
ground caused by instabilities in the CS number density pro-
duced in the pulsed-discharge supersonic expansion. This CS
production noise can be largely removed by using the
mmOPS technique. When the vertically polarized pump laser
is resonant with a CS electronic transition, it creates an an-
isotropic sample of CS molecules by preferentially pumping
certain MJ states. If one of the levels of the millimeter-wave
transition is in common with the pump-laser transition, then
the vertical and horizontal components of the millimeter-
wave probe are differentially absorbed, causing a polariza-
tion rotation of the millimeter waves. By using a set of
crossed polarizers, the unrotated millimeter-wave radiation
can be blocked, while only the rotated millimeter-wave ra-
diation strikes the detector. Thus, the polarization-detected
experiment can have a background level that is limited only
by the imperfect extinction of the polarizers and the birefrin-
gence of the millimeter-wave transmission optics. When the
polarizers are perfectly crossed, all mmOPS signals should
have the same sign because millimeter-wave radiation strikes
the detector only when the optical pump transition originates
from one of the linked levels of the millimeter-wave transi-
tion. The signal levels in this case are often too small to be
observed with the noise level of the current detector. As in
optical polarization-based experiments,17 however, the
signal-to-background ratio can be improved by imperfectly
crossing the polarizers to heterodyne the weak polarization-
rotation signal field with the much stronger nonrotated field.
Here, the spectra are collected at relatively large uncrossing
angles ͑ϳ5°͒ to obtain the best signal-to-background ratio.
Although the background level at 5° is approximately twice
the background level when the polarizers are perfectly
crossed, the cross term between the signal and carrier
millimeter-wave electric fields is much larger, which ulti-
mately increases the signal-to-background ratio. The hetero-
dyne term leads to both positive and negative features in the
mmOPS spectrum. In contrast to the mmODR spectra, how-
ever, the sign and intensity of the observed features contain
information about the rotational branch excited by the optical
field. A detailed theoretical analysis ͑in the nonperturbative
limit͒ of the expected intensities is currently in progress.18
The spectra in Fig. 2 have been normalized so that the
strongest transition in each spectrum has the same peak in-
tensity. The baseline in the region of 39 829 cm−1 is ex-
FIG. 3. mmOPS spectra of nominally spin-forbidden electronic transitions
to triplet states of CS, which borrow their intensity via spin-orbit interac-
tions with the A 1⌸ state. The millimeter-wave frequency is locked to the CS
X 1⌺+
͑
=0,J =2-1͒ rotational transition and the laser is scanned over the
v
Љ Љ
͑a͒ e 3⌺− −X 1⌺+ ͑2-0͒ and the ͑b͒ d 3⌬−X 1⌺+ ͑6-0͒ bands. The e 3⌺−
͑
v
Ј
=2͒ and the d 3⌬ ͑ =6͒ states have 17% and 1% nominal A 1⌸ character,
v
Ј
respectively, for the rotational levels accessed ͑Ref. 13͒. Each point is an
average of 20 laser shots.
panded to show that baseline noise is suppressed by a factor
of 4 for mmOPS relative to mmODR. The enhancement in
sensitivity ͑signal-to-background ratio͒ afforded by the
mmOPS technique has enabled us to record nominally spin-
forbidden optical transitions into triplet states of CS, as
shown in Fig. 3. Although the sensitivity enhancement of
mmOPS is largely due to the reduction of the CS production
noise present in the CS ground-state mmODR signal, it is
also improved because the millimeter-wave power incident
to the molecular beam can be increased without saturating
the millimeter-wave detector. In the absorption-detected
technique the power of the Gunn oscillator is attenuated by
approximately 15 dB to 600 ␮W to avoid saturation of the
detector/preamplifier, while in the polarization-detected tech-
nique the full power of the oscillator ͑nominally 18 mW at
98 GHz͒ is incident to the molecular beam, but is attenuated
by the crossed polarizers to Ͻ1 mW on the detector.
To demonstrate the additional capabilities of the mmOPS
technique, we have measured a pure rotational transition in
one of the optically populated triplet states. Figure 4 shows
the mmOPS spectrum recorded with the laser populating the
J =1, N =1 level of the e 3⌺− ͑ =2͒ state. The trace is the
v
Ј
Ј
Ј
average of four scans, each recorded by stepping the Gunn
oscillator output in 50 kHz steps and averaging the bolom-
eter output for ten laser shots at each frequency. The center
frequency of the triplet rotational transition J =2, N =2
Ј
Ј
←J =1, N =1 is determined to be 76 229.027͑20͒ MHz.
Ј
Ј
The measured linewidth is 1.3 MHz, which is significantly
broader than the ground-state linewidth ͓ϳ240 kHz full
width at half maximum ͑FWHM͔͒. We attribute the addi-
tional linewidth to the radiative lifetime of the triplet state,
which we expect to be ϳ1.2 ␮s, assuming that the lifetime is
dominated by the ϳ17% singlet character13 of the populated
“triplet” level and the 198 ns lifetime for the A 1⌸ ͑v=1͒
state.19
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141102-4
Steeves et al.
J. Chem. Phys. 123, 141102 ͑2005͒
cay rate. In addition, the development of sensitive new tech-
niques such as millimeter-wave-detected mmOPS makes it
possible to observe and identify optical transitions in com-
plex environments, such as discharges, where traditional
one-color measurements can be limited by spectral conges-
tion. We plan to extend the millimeter-wave-detected
mmOPS technique to measure electric and magnetic mo-
ments of electronic and highly vibrationally excited states
with Stark and Zeeman spectroscopy.
The authors are grateful to Carl Gottlieb and Kelly Hig-
gins for the use of their equipment. One of the authors
͑A.H.S͒ acknowledges support from the Army Research Of-
fice under a National Defense Science and Engineering
Graduate Fellowship. This work was supported by the Office
of Basic Energy Sciences of the U.S. Department of Energy
͑DE-FG0287ER13671͒.
FIG. 4. Pure rotational mmOPS spectrum of an excited triplet state. The
laser frequency is fixed to populate the J =1, N =1 level of the e 3⌺− state
Ј
Ј
via the J =1, N =1←J =1 transition of the e 3⌺− −X 1⌺+ ͑2-0͒ vibrational
Ј
Ј
Љ
band. The center frequency of the pure rotational transition J =2, N =2
Ј
Ј
←J =1, N =1 in the electronically excited state is 76 229.027͑20͒ MHz.
Ј
Ј
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In contrast to millimeter-wave transitions in the CS
ground state, measurements of mmODR signals in the ex-
cited electronic state are not compromised by fluctuations in
the background absorption of ground-state CS molecules.
Rather, the main contributor to the baseline noise is the
millimeter-detector noise, which is larger than the amplitude
noise of the millimeter-wave source. Because the most sig-
nificant advantage of the polarization-detected technique
with the current millimeter-wave detector is the suppression
of the CS source noise, the excited-state mmODR and
mmOPS upper state spectra are nearly identical in quality. In
addition to detector noise, both the mmODR and mmOPS
techniques are artificially limited by the bandwidth
͑5 Hz–500 kHz͒ of the millimeter-wave detector. This limi-
tation attenuates transient nutation signals with periods
shorter than 2 ␮s and is particularly disadvantageous for the
mmOPS technique, which would otherwise use the full
power of the millimeter-wave source to induce the fastest
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