IEEE JOURNAL OF QUANTUM ELECTRONICS 1 New far-infrared laser emissions from optically 18 pumped CH2DOH, CHD2OH, and CH3 OH M. McKnight, P. Penoyar, M. Pruett, N. Palmquist, S. Ifland, and M. Jackson

18 Abstract—An optically pumped molecular laser system utiliz- Similarly, CH3 OH has been found to generate about 110 FIR ing a transverse or zig-zag pumping geometry of the far-infrared laser emissions [24]–[28]. Unlike CH2DOH and CHD2OH, laser medium has enabled the reinvestigation of the CH2DOH, 18 spectroscopic assignments for numerous FIR laser emissions CHD2OH, and CH3 OH isotopic forms of methanol for wave- lengths above 100 µm. With this system, 28 far-infrared laser have successfully been performed [25], [27], [29]–[32]. emissions have been discovered with wavelengths ranging from For this investigation, an optically pumped molecular laser 117.2 to 744.7 µm. Along with the wavelength, each laser emission has been used. This system consists of a CO2 laser that is reported with its optimal operating pressure, polarization with excites the far-infrared laser medium using a transverse or zig- respect to the CO2 pump laser, and relative intensity. Three of the 18 zag pumping geometry. Although originally developed for use laser emissions generated by CH3 OH support the spectroscopic assignments of FIR laser transitions originally proposed through with pulsed lasers [33], this pumping geometry has also been 18 combination-difference loops. A fourth CH3 OH laser emission successfully implemented with cw pump lasers [34]. With this should contribute to another, incomplete FIR laser scheme. experimental system, over three hundred FIR laser emissions 18 have been observed [35]–[37]. This includes nearly 140 FIR Index Terms—CH2DOH, CHD2OH, CH3 OH, optically pumped molecular laser. laser emissions generated by optically pumped CH2DOH, 18 CHD2OH, and CH3 OH of which 28 have been observed for the first time. These new laser lines range in wavelength I.INTRODUCTION from 117.2 to 744.7 µm and should prove useful in assisting ETHANOL and its isotopic forms are the richest, with the spectroscopic assignment of FIR laser schemes and M most prolific media for optically pumped molecular enhance a variety of applications that use cw optically pumped lasers [1]–[3]. These molecules are particularly efficient for lasers as a source of terahertz radiation. laser pumping because several of their vibrational bands, including the CO-stretching, in-plane CH3-rocking, and CH3- II.EXPERIMENTAL DETAILS deformation infrared bands, tend to coincide with the 9 and 10 µm emission bands from the carbon dioxide (CO2) laser. HE optically pumped molecular laser system used for this Additionally, the infrared spectra of these molecules are ex- T investigation has been described in detail previously [35]. tremely dense due to their slight asymmetry, large permanent FIR laser emissions were detected using a liquid helium cooled electric dipole moments, and hindered internal rotation [4], Si bolometer, operated at 4 K. Once a laser emission was [5]. These properties lead to numerous absorption lines being detected, the operating pressure was adjusted until its output available to serve as possible pumping candidates by the CO2 power was maximized. Laser wavelengths were measured with laser. an uncertainty of  0.5 µm by traversing at least 20 adjacent Far-infrared (FIR) laser radiation generated by CH2DOH longitudinal modes for a particular FIR laser emission. This and CHD2OH was first obtained by Ziegler and Durr¨ [6]. was accomplished using a calibrated micrometer capable of In all, about 90 FIR laser emissions have been found for adjusting one end mirror. Figure 1 illustrates a portion of a CH2DOH [7]–[13] while nearly 170 have been observed for typical cavity scan, with the intensity plotted as a function of CHD2OH [11], [13]–[19]. A challenge with these isotopic decreasing FIR laser cavity length. A set of absorbing filters, forms of methanol is that deuterium substitution results in calibrated with wavelength, attenuated the CO2 laser radiation a greater molecular asymmetry creating an asymmetric-top and helped distinguish different FIR laser wavelengths. The asymmetric-frame molecule [20]. This increases the complex- polarization of the FIR laser emission relative to the CO2 ity in the spectrum whose features include multiple interacting pump laser was determined using a 1000 line/mm wire-grid levels. Despite these challenges, a handful of FIR laser transi- polarizer. The CH2DOH and CHD2OH samples were 98% D 18 18 tions have successfully been assigned for CH2DOH [21]–[23]. enriched while CH3 OH was 95% O enriched. All observed laser emissions were compared against known lines generated This material is based upon work supported by the National Science Foun- by optically pumped CH3OH. dation (Award No. 0910935) and the Washington Space Grant Consortium (Award No. NNX10AK64H). M. McKnight P. Penoyar, M. Pruett, and M. Jackson are with the De- III.RESULTS partment of Physics, Central Washington University, Ellensburg, WA 98926. E-mail: [email protected]. HE new FIR laser emissions from optically pumped N. Palmquist is with The Whiting School of Engineering, Johns Hopkins 18 CH2DOH, CHD2OH, and CH3 OH are listed in Table University, Baltimore, MD 21218. T S. Ifland is with the Department of Physics, Edmonds Community College, I, arranged in order of the CO2 pump line. Laser wavelengths Lynnwood, WA 98026. are reported as the average of at least four independent sets of IEEE JOURNAL OF QUANTUM ELECTRONICS 2

TABLE I 762.5 ȝm 744.7 m ȝ NEW FAR-INFRAREDLASEREMISSIONSFROMOPTICALLYPUMPED 18 CH2DOH,CHD2OH, AND CH3 OH

CO2 Wavelength Operating Relative Relative Pump (µm) Pressure (Pa) Polarization Intensity

Intensity CH2DOH 9P 02 220.0 12.7 ⊥ M 9P 18 744.7 14.7 ⊥ W 9P 44 117.2 17.3 ⊥ M 10R46 321.9 14.7 ∥ M 23.1 mm Micrometer Position 19.1 mm 10R32 224.0 10.7 ⊥ VW 10HP 09 291.8 9.3 ∥ VW ∥ Fig. 1. Portion of a typical cavity scan illustrating adjacent longitudinal 322.9 9.3 VW 10P 52 438.2 11.3 Both M modes of the FIR laser emission from optically pumped CH2DOH. This scan shows the new 744.7 µm and known 762.5 µm [7] laser emissions CHD2OH generated using the 9P 18 CO2 pump. A decrease in the micrometer position corresponds to a decrease in the length (mirror-to-mirror separation) of the 9R14 529.9 12.0 Both M ∥ FIR laser cavity. 9P 04 311.5 11.3 M 472.3 13.3 Both S 10R50 346.2 13.3 ∥ W 10R48 422.1 8.0 Both M measurements. The relative intensity of the FIR laser output 557.1 8.0 Both M 10R44 225.9 24.0 ⊥ S was labeled as either S, M, W, or VW corresponding to power 518.0 18.7 ∥ M ranges from 1-0.1 mW, 0.1-0.01 mW, 0.01-0.001 mW, and 10R26 272.4 10.0 ∥ VW ∥ below 1 µW, respectively. For comparison, the 118.8 µm line 483.3 8.0 VW 10R16 426.0 7.5 ⊥ S of methanol was observed with this system to be VVS with a 10P 02 152.3 13.3 ⊥ VW power slightly above 10 mW when using a 15 W, 9P 36 CO2 18 pump. CH3 OH 9R38 433.3a 13.3 ∥ M Table I includes the polarization of each FIR laser emission 676.4b 9.3 Both M ∥ relative to its respective CO2 pump. In most cases, only one 9R30 643.8 13.3 M c ∥ polarization was observed to dominate, either a polarization 9P 14 209.9 20.0 M 9P 18 359.3d 11.3 ∥ M parallel or perpendicular to the CO2 pump laser. For situ- 10R30 433.8 12.0 ∥ W ations where no dominant polarization was observed, both 10R02 274.2 30.7 ∥ S polarizations have been listed. For a number of laser emis- 526.0 20.0 ⊥ S sions observed using this far-infrared laser cavity, a dominant a Wavelength was predicted to be 433.36 µm [31]. b polarization emerged only after the output coupler was slightly Wavelength was predicted to be 676.54 µm [31]. c µ repositioned away from the location where maximum FIR laser Wavelength was predicted to be 209.82 m [29], [32]. d Wavelength was predicted to be 359.25 µm [31]. power occurred. Therefore, the observed polarization appeared to be strongly dependent on the position of the output coupler and hence may be cavity dependent. are now discussed. The 744.7 µm laser emission from CH2DOH was generated by 9P 18 at nearly the same offset frequency as the 762.5 µm The energy level and transition structure associated with the line, both of which were observed on the edge of the CO2 9R38 CO2 laser pump is shown in Fig. 2 [31]. The 286.3 µm laser’s gain curve (≈ 45  5 MHz). However, this offset was line, also observed during this investigation, was first detected different from the offset frequency used to obtain the 167.5 and by Ioli et al. [24]. Based on the analysis of this laser emission, 396.0 µm laser emissions also generated by this CO2 pump LB, two FIR laser lines were subsequently predicted for this − Vrock → line [6], [7]. Additionally, two lines observed from CH2DOH FIR laser scheme: (LC) = (n = 0, K = 2, J = 15) A − have been tentatively identified as being generated by the (n = 0, K = 2, J = 14)Vrock A with a wavelength of 433.36 − Vrock → 10HP 09 CO2 pump. This pump line was located between the µm and (LA) = (n = 0, K = 2, J = 15) A (n = 0, K Vrock + 10P 44 and 10P 46 regular CO2 band emissions. The known = 1, J = 15) A with a wavelength of 676.54 µm. The 512.8 µm laser emission from CHD2OH obtained with the newly observed FIR laser lines at 433.3 and 676.4 µm appear 9P 04 CO2 pump [14] was not observed in this investigation. to support the proposed spectroscopic assignments. Rather, a 518.3 µm laser line was observed on this CO2 Figure 3 illustrates the energy level and transition structure pump under similar operating conditions. While the 518.3 µm associated with the 9P 18 CO2 laser pump [31]. One of the emission could be a new line, it is possible the previously FIR laser emissions resulting from the analysis of this laser V reported wavelength had its last two digits transposed. scheme is the (LB) = (n = 0, K = 2, J = 14) rock E1 → (n = 0, 18 V For CH3 OH, the spectroscopic assignments of numerous K = 1, J = 13) rock E1 transition with a predicted wavelength FIR laser schemes have been conducted [25], [27], [29]– of 359.25 µm. This laser emission has been observed with a [32]. These studies include a thorough description of the measured wavelength of 359.3 µm, supporting the proposed spectroscopic notation and analysis used to perform these spectroscopic assignment. The only other FIR laser emission assignments. Additionally, a number of FIR laser lines have observed with this CO2 pump was the known LC = 465.5 µm been predicted as a result of these studies, several of which line [24]. While the offset frequency for these lines is predicted IEEE JOURNAL OF QUANTUM ELECTRONICS 3

(n =0, K =2, A)Vrock Vrock + (n =0, K =1, A) - 15 The 182.2 µm line has been frequency measured by Carelli - (LA ) 15 + (L ) ++ and co-workers [26] and appears to be in excellent agreement C - 14 - L 14 + B with the calculated frequency [31]. The remaining transitions (n =1, K =3, A)V =0 predicted at this offset [32], including the known 214.2 µm - + 9R 38 J 14 line [24], were not observed. Depending on the absorption

R characteristics of these lines this may not be surprising,

2

R(14)1089.00121

R(13)1087.54709 O Q(14)247.90969 particularly when coupled with the +70 MHz offset frequency J C 9 381089.0011

R(14)1091.64908 R(13)1090.57230 ] + - 14 - Q(14)[17.42893 R(13)269.53098 necessary to pump these emissions which is beyond our tuning 14 + ++- 13 range. Observation of the fourth 209.9 µm line is interesting - 13 + R(13)36.52863 (n =0, K =2, A)V =0 since it appears to align itself with the assignment originally (n =0, K =1, A)V =0 proposed for this system (n = 0, K = 2, J = 36)Vco A+ → V + Fig. 2. Energy level and transition structure for the FIR laser system of (n = 0, K = 3, J = 35) co A [29] having a loop-calculated 18 CH3 OH optically pumped by the 9R38 CO2 laser [31]. wavelength of 209.82 µm [32]. The observation of this laser emission should motivate a re-analysis of this particular laser (n =0, K =2,E)Vrock 1 scheme whose complexity arises from the very weak spectral (n =0, K =1,E)Vrock (L ) 14 1 A lines associated with the energy levels that constitute this high 14 L C 13 J FIR laser system. (L ) 13 B IV. CONCLUSION HE CH DOH, CHD OH, and CH 18OH isotopologues 9P 18-67MH z 2 2 3 T of methanol have been used to generate nearly 140 P optically pumped laser emissions in the far-infrared region,

2 J

P(14)1050.30985

P(15)1048.65822 O twenty eight of which were observed for the first time. The

J C 9 181048.6608

P(15)1043.68501

P(14)1045.26072 laser emissions discovered during this investigation demon- 162 15 Q(15)1.248 strate this system’s effectiveness in generating far-infrared 15 3828 24.4 14 radiation above 100 µm. They are also indicative of the R(14) n K V =0 system’s sensitivity in detecting weak laser radiation. These 14 .227093 ( =0, =2,E)1 n K V =0 Q(14)1 ( =0, =1,E)1 new laser lines help expand the important role of optically pumped molecular lasers in experimental applications where Fig. 3. Energy level and transition structure for the FIR laser system of they serve as a THz radiation source for spectroscopic investi- 18 CH3 OH optically pumped by the 9P 18 CO2 laser at an offset of −67 MHz [31]. gations, as in [39]. Additionally, the observations of FIR laser emissions are critical in assessing the analysis of complex spectra recorded in the infrared through microwave regions. to be −67 MHz and beyond the system’s tuning range, they Such are the contributions of this investigation by confirming were both observed to be on the edge of the CO2 laser’s gain the spectroscopic assignments associated with several laser 18 curve. Observation of a FIR laser emission whose offset is schemes for optically pumped CH3 OH. As the theoretical centered at a frequency outside the system’s tuning range is formalism that has been developed for methanol is applied possible and depends on the absorption characteristics of the toward more complex isotopologues, FIR laser emissions can FIR laser medium coupled with how well it overlaps with assist in confirming the spectroscopic assignments of transi- the Doppler broadened profile of the CO2 pump laser [38]. tions by providing connections among the excited vibrational The remaining predicted laser emission (LA) = 1607.36 µm state levels that are often directly inaccessible from absorption may be challenging to detect with this experimental system spectra. for several reasons including its long wavelength, absorption characteristics, and the need to use an offset whose center REFERENCES frequency is predicted to be beyond the system’s tuning range. Finally, assignments of FIR laser lines generated by the [1] D. Pereira, J. C. S. Moraes, E. M. Telles, A. Scalabrin, F. Strumia, A. Moretti, G. Carelli, and C. A. Massa, “A review of optically pumped far- 9P 14 CO2 laser have been performed by Lees and co-workers infrared laser lines from methanol isotopes,” Int. J. Infrared Millimeter using two different offset frequencies, −20 and +70 MHz [29], Waves, vol. 15, no. 1, pp. 1-44, Jan. 1994. [31], [32]. This CO laser line is particularly interesting since [2] S. C. Zerbetto and E. C. C. Vasconcellos, “Far infrared laser lines 2 produced by methanol and its isotopic species: A review,” Int. J. Infrared it has the ability to pump IR transitions in both the CO-stretch Millimeter Waves, vol. 15, no. 5, pp. 889-933, May 1994. and CH3-rocking bands. For the −20 MHz offset, we observed [3] G. Moruzzi, B. P. Winnewisser, M. Winnewisser, I. Mukhopadhyay, and both known laser emissions [24] to have wavelengths of 482.1 F. Strumia, Microwave, Infrared and Laser Transitions of Methanol: Atlas of Assigned Lines from 0 to 1258 cm−1, CRC Press, Boca Raton, 1995. and 655.0 µm which appear to be in agreement with their [4] L.-H. Xu, R. M. Lees, E. C. C. Vasconcellos, S. C. Zerbetto, L. R. calculated wavelengths of 482.04 and 654.73, respectively Zink, and K. M. Evenson, “Methanol and the optically pumped far-infrared [31]. Two FIR laser emissions were also observed at a different laser,” IEEE J. Quantum Electron., vol. 32, no. 3, pp. 392-399, Mar. 1996. [5] I. Mukhopadhyay, “Spectroscopy of methanol and its application to offset on the edge of the CO2 laser’s gain curve. They were optically pumped THz FIR lasers,” Int. J. Infrared Millimeter Waves, the known 182.2 µm line [24] and the new 209.9 µm line. vol. 24, no. 5, pp. 709-755, May 2003. IEEE JOURNAL OF QUANTUM ELECTRONICS 4

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Matthew Pruett is currently working toward his Bachelor of Science degree in physics at Central Washington University and his Bachelor of Science degree in mechanical engineering at Washington State University.

Nathan Palmquist is currently working toward his Bachelor of Science degree in materials science and engineering at Johns Hopkins University.

Sumaya Ifland is currently working toward her Associate of Science degree at Edmonds Community College.

Michael Jackson received his Bachelor of Science degree in physics and mathematics from SUNY– Oswego in 1992 and his PhD in physics from New Mexico State University in 1998. He is a Professor in the Department of Physics at Central Washington University. His research interests include infrared and far-infrared lasers with their application to molecular spectroscopy.