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Electronic transition CN laser
Volume 18, number 3
ELECTRONIC
OPTICS COMMUNICATIONS
TRANSITION
August 1976
CN LASER*
C.R. QUICK Jr. and Curt WITTIG Department of Electrical Engineering, University of Southern University Park, Los Angeles, California 90007, USA
California,
and J.B. LAUDENSLAGER Jet Propulsion Laboratory,
Pasadena, California 91103,
USA
Received 22 April 1976
An electric discharge pumped CN laser oscillating on the A*H-X2 Z system is reported. Peak power of 20 kW in a 150 nsec fwhm pulse is obtained using a simple longitudinal electric discharge in several mmHg of HCN. Oscillation occurs on the (0,l) and (0,2) bands at - 1.42 and 2.0pm respectively
1. Introduction Radiative emission from CN has been the basis for considerable experimental investigation in the past. Studies have been concerned with areas such as reaction kinetics [l] , bond dissociation energies [2], energy partitioning in photodissociation fragments [3-51, electron impact phenomena [6-81, and many others. Most of this work has been concerned with emission from the first two electronically excited states of the CN species (A2 Il and B2 E’), commonly referred to as the red and violet systems of CN respectively. Vibrational excitation of the electronic ground state (X2x+) of CN was first observed by Basco et al. in flash photolysis experiments [3]. Pollack [9] subsequently observed stimulated emission at - 5.2 pm from the flash photolysis of C2N2. The. first observation of stimulated emission between electronic states of CN was by West and Berry [lo]. They observed that photodissociation of selected CN containing compounds resulted in production of CN(A211(u=O)) as a primary photo-
chemical product. Laser oscillation was observed on the (O,O), (0, l), and (0,2) vibrational bands of the A+X system at 1 .l, 1.4, and 2.0pm respectively. They also identified the pumping mechanism of the vibrational CN laser as lntra-molecular electronic-vibrational (E-V) energy transfer from CN(A211 (u= 0)) to CN(X2C+(u)). The CN species is an attractive laser candidate for the A + X system because of the 7psec radiative lifetime of the A211 state and favorable Franck-Condon factors. The observation of spontaneous emission from the B and A states of CN in numerous chemical reactions has led researches to pursue the possibility of a chemically pumped electronic transition CN laser [ 111. To date, these efforts have been unsuccessful. This communication describes a 20 kW A+X CN laser excited by a simple longitudinal discharge through HCN vapor. Possible excitation mechanisms and potential laser efficiency are also discussed.
2. Experimental * Research supported by the Advanced Research Projects Agency of the Department of Defense, monitored by ONR under Contract No. N00014-75Ca567, and the Caltech Present’s Fund (NASA Contract NAS-7-100).
268
The experimental arrangement is straightforward. The laser tube is constructed of Pyrex with dimensions 1 .l cm I.D. X 85 cm active length, and is sealed at the
Volume 18, number 3
OPTICS COMMUNICATIONS
August 1976
ends with Pyrex Brewster windov j. Voltage pulses of 20-75 kV are supplied from a Malx generator at energies of 1-36 J. Laser pulse shapes are measured with an InSb PEM detector (risetime x 10 nsec), and energies are measured with a thermopile. Spectra are measured with a 0.5 m grating monochromator. Laser performance is not critical to optical cavity configuration. Typically, a ZnSe beam splitter, in conjunction with two Au coated cavity mirrors, is used for output coupling. Chemicals were obtained from commercial sources (HCN, Fumico; C,N,, Matheson; ICN and BrCN, Baker; He and Ar, Airco). The cyanide compounds were degassed at 77 K and used without further purification. Mixtures of the cyanides with He and Ar were prepared in a mixing bulb and then introduced into the laser cavity. The effect of impurities is minimal on the time scales of interest in this work (
3. Resutls The production of CN(A2H) in sufficient density to produce laser oscillation in the A-+X system occurs in pulsed electric discharges containing roughly 0.6 to 6 torr of HCN. Higher HCN pressures are difficult to discharge with the present experimental arrangement. Laser oscillation is readily detected at the minimum input energy, which is 1 J. CN bearing compounds other than HCN (such as C2N2, BrCN, and ICN) failed to produce stimulated emission under any conditions using the apparatus described above. Spectral analysis of the laser output revealed two bands in the A-+X system: the (0,l) and (0,2) bands at - 1.42 and 2.0 pm respectively. Both P and Q branches are active in each band, the Q branches being stronger. The maximum laser energy was 3 mJ, which corresponds to approximately 20 kW of peak power. Approximately 30% of the laser energy is contained in the (0,l) band, with the remaining 70% in the (0,2) band. Maximum energy was obtained with a 1 to 6 mixture of HCN to He, at a total pressure of 20 torr. The input energy for this case was 36 J in a 60 kV pulse. Output coupling was 50% using the ZnSe beam splitter. The time development of the pump-current pulse and the laser pulse, under maximum energy conditions, is shown in fig. 1. The current pulse has a rise-time of -0.5 psec and undergoes considerable ringing, finally
Fig. 1. In (a), is shown the exciting current pulse from the Marx generator. The peak current is 3000 A. In (b), is shown the laser emission on the A-+ X transitions. Note the difference in time scales.
dying off after -4psec. Peak current is about 3000 A. The laser pulse starts - 120 nsec. into the current pulse and lasts - 180 nsec (fwhm). All laser oscillation occurs within the rise-time of the current pulse. The addition of the He to the HCN vapor improves discharge uniformity and essentially doubles the laser energy by increasing the pulse duration. The peak power remains about the same with and without He. Ar also enhances the energy output but to a lesser degree than He.
4. Discussion West and Berry [lo] have shown that the primary product of the W photolysis of HCN is CN(A211(u=O)). 269
Volume 18, number 3
OPTICS COMMUNICATIONS
The specific production of this state occurs via a state of HCN that predissociates. The similarity between the results of West and Berry and the present work suggests that the same excited state of HCN may be involved in both cases. Thus, a possible pumping mechanism is: HCNte+HCN*te,
August 1976
(1)
where electron energy is the mean energy of electrons which are capable of producing HCN *. This number is high (- 5-10%) and suggests that a high power efficient device may be possible. Because of the long spontaneous emission lifetime (7 psec) of the A2 JI state, it will also be possible to Q-switch the laser under conditions where collisional quenching of A2Jl is minimized .
(2)
Acknowledgement
followed by predisaxiation
HCN * -
.
H(lS) + CN(A2fi)
Korol et al. [8] have shown that electron impact excitation of HCN results in the production of both CN(A2TI) and CN(B24Z’). Their results are consistent with our own observations, and also suggest the possibility of CN lasers oscillating on the B+A and B-+X transitions. The inability of our device to oscillate on the A+X (0,O) band is not surprising in light of the short spontaneous lifetime (60 nsec) of the B+X transitions, and the large cross section for production of CN(B21?(u= 0)) via electron impact excitation of HCN [8]. Since the u = 0 progression dominates the B-+X transitions, the production of CN(B2Z+(u=O)) quickly results in CN(X2 ZS’(u = 0)). Also, the production of CN(X2Z’(u= 0)) via dissociative attachment of low energy electrons by HCN is a distinct possibility. Although the maximum efficiency of our device (-0.1%) is low, the probable pumping mechanism suggests that a more efficient device is possible by suitably controlling the electron energy. The quantum efficiency of the laser is given by: quantum
270
efficiency =
h v (laser) electron energy
’
(3)
The authors acknowledge the technical assistance of H.R. Owen and J. Emerson, and the loan of equipment by W.H. Steier.
References 111J.C. Boden and B.A. Thrush, Proc. Roy. Sot. A305 (1968) 107.
PI D.D. Davis and H. Okabe, J. Chem. Phys. 49 (1968) 5526. [31 N. Basco, J.E. Nicholas, R.G.W. Norrish and W.H.J. Vickers, Proc. Roy. Sot. A272 (1963) 147. 141 W.M. Jackson, J. Chem. Phys. 61 (1974) 4177. 151 A. Mele and H. Okabe, J. Chem. Phys. 51 (1969) 4798. VI V.I. Korol and S.M. Kishko, Opt. Spectrosc. 33 (1972) 205. [71 I. Tokue, T. Urisu and K. Kuchitsu, J. Photochem. 3 (1974/ 75) 273. PI V.I. Korol and S.M. Kishko, Opt. Spectrosc. 38 (1975) 486. PI M.A. Pollack, Appl. Phys. Lett. 9 (1966) 230. [lOI G. West and M. Berry, J. Chem. Phys. 61 (1974) 4700. [Ill H.P. Broida, Appl. Opt. Suppl. on Chemical Lasers (1965) 105.
ELECTRONIC
OPTICS COMMUNICATIONS
TRANSITION
August 1976
CN LASER*
C.R. QUICK Jr. and Curt WITTIG Department of Electrical Engineering, University of Southern University Park, Los Angeles, California 90007, USA
California,
and J.B. LAUDENSLAGER Jet Propulsion Laboratory,
Pasadena, California 91103,
USA
Received 22 April 1976
An electric discharge pumped CN laser oscillating on the A*H-X2 Z system is reported. Peak power of 20 kW in a 150 nsec fwhm pulse is obtained using a simple longitudinal electric discharge in several mmHg of HCN. Oscillation occurs on the (0,l) and (0,2) bands at - 1.42 and 2.0pm respectively
1. Introduction Radiative emission from CN has been the basis for considerable experimental investigation in the past. Studies have been concerned with areas such as reaction kinetics [l] , bond dissociation energies [2], energy partitioning in photodissociation fragments [3-51, electron impact phenomena [6-81, and many others. Most of this work has been concerned with emission from the first two electronically excited states of the CN species (A2 Il and B2 E’), commonly referred to as the red and violet systems of CN respectively. Vibrational excitation of the electronic ground state (X2x+) of CN was first observed by Basco et al. in flash photolysis experiments [3]. Pollack [9] subsequently observed stimulated emission at - 5.2 pm from the flash photolysis of C2N2. The. first observation of stimulated emission between electronic states of CN was by West and Berry [lo]. They observed that photodissociation of selected CN containing compounds resulted in production of CN(A211(u=O)) as a primary photo-
chemical product. Laser oscillation was observed on the (O,O), (0, l), and (0,2) vibrational bands of the A+X system at 1 .l, 1.4, and 2.0pm respectively. They also identified the pumping mechanism of the vibrational CN laser as lntra-molecular electronic-vibrational (E-V) energy transfer from CN(A211 (u= 0)) to CN(X2C+(u)). The CN species is an attractive laser candidate for the A + X system because of the 7psec radiative lifetime of the A211 state and favorable Franck-Condon factors. The observation of spontaneous emission from the B and A states of CN in numerous chemical reactions has led researches to pursue the possibility of a chemically pumped electronic transition CN laser [ 111. To date, these efforts have been unsuccessful. This communication describes a 20 kW A+X CN laser excited by a simple longitudinal discharge through HCN vapor. Possible excitation mechanisms and potential laser efficiency are also discussed.
2. Experimental * Research supported by the Advanced Research Projects Agency of the Department of Defense, monitored by ONR under Contract No. N00014-75Ca567, and the Caltech Present’s Fund (NASA Contract NAS-7-100).
268
The experimental arrangement is straightforward. The laser tube is constructed of Pyrex with dimensions 1 .l cm I.D. X 85 cm active length, and is sealed at the
Volume 18, number 3
OPTICS COMMUNICATIONS
August 1976
ends with Pyrex Brewster windov j. Voltage pulses of 20-75 kV are supplied from a Malx generator at energies of 1-36 J. Laser pulse shapes are measured with an InSb PEM detector (risetime x 10 nsec), and energies are measured with a thermopile. Spectra are measured with a 0.5 m grating monochromator. Laser performance is not critical to optical cavity configuration. Typically, a ZnSe beam splitter, in conjunction with two Au coated cavity mirrors, is used for output coupling. Chemicals were obtained from commercial sources (HCN, Fumico; C,N,, Matheson; ICN and BrCN, Baker; He and Ar, Airco). The cyanide compounds were degassed at 77 K and used without further purification. Mixtures of the cyanides with He and Ar were prepared in a mixing bulb and then introduced into the laser cavity. The effect of impurities is minimal on the time scales of interest in this work (
3. Resutls The production of CN(A2H) in sufficient density to produce laser oscillation in the A-+X system occurs in pulsed electric discharges containing roughly 0.6 to 6 torr of HCN. Higher HCN pressures are difficult to discharge with the present experimental arrangement. Laser oscillation is readily detected at the minimum input energy, which is 1 J. CN bearing compounds other than HCN (such as C2N2, BrCN, and ICN) failed to produce stimulated emission under any conditions using the apparatus described above. Spectral analysis of the laser output revealed two bands in the A-+X system: the (0,l) and (0,2) bands at - 1.42 and 2.0 pm respectively. Both P and Q branches are active in each band, the Q branches being stronger. The maximum laser energy was 3 mJ, which corresponds to approximately 20 kW of peak power. Approximately 30% of the laser energy is contained in the (0,l) band, with the remaining 70% in the (0,2) band. Maximum energy was obtained with a 1 to 6 mixture of HCN to He, at a total pressure of 20 torr. The input energy for this case was 36 J in a 60 kV pulse. Output coupling was 50% using the ZnSe beam splitter. The time development of the pump-current pulse and the laser pulse, under maximum energy conditions, is shown in fig. 1. The current pulse has a rise-time of -0.5 psec and undergoes considerable ringing, finally
Fig. 1. In (a), is shown the exciting current pulse from the Marx generator. The peak current is 3000 A. In (b), is shown the laser emission on the A-+ X transitions. Note the difference in time scales.
dying off after -4psec. Peak current is about 3000 A. The laser pulse starts - 120 nsec. into the current pulse and lasts - 180 nsec (fwhm). All laser oscillation occurs within the rise-time of the current pulse. The addition of the He to the HCN vapor improves discharge uniformity and essentially doubles the laser energy by increasing the pulse duration. The peak power remains about the same with and without He. Ar also enhances the energy output but to a lesser degree than He.
4. Discussion West and Berry [lo] have shown that the primary product of the W photolysis of HCN is CN(A211(u=O)). 269
Volume 18, number 3
OPTICS COMMUNICATIONS
The specific production of this state occurs via a state of HCN that predissociates. The similarity between the results of West and Berry and the present work suggests that the same excited state of HCN may be involved in both cases. Thus, a possible pumping mechanism is: HCNte+HCN*te,
August 1976
(1)
where electron energy is the mean energy of electrons which are capable of producing HCN *. This number is high (- 5-10%) and suggests that a high power efficient device may be possible. Because of the long spontaneous emission lifetime (7 psec) of the A2 JI state, it will also be possible to Q-switch the laser under conditions where collisional quenching of A2Jl is minimized .
(2)
Acknowledgement
followed by predisaxiation
HCN * -
.
H(lS) + CN(A2fi)
Korol et al. [8] have shown that electron impact excitation of HCN results in the production of both CN(A2TI) and CN(B24Z’). Their results are consistent with our own observations, and also suggest the possibility of CN lasers oscillating on the B+A and B-+X transitions. The inability of our device to oscillate on the A+X (0,O) band is not surprising in light of the short spontaneous lifetime (60 nsec) of the B+X transitions, and the large cross section for production of CN(B21?(u= 0)) via electron impact excitation of HCN [8]. Since the u = 0 progression dominates the B-+X transitions, the production of CN(B2Z+(u=O)) quickly results in CN(X2 ZS’(u = 0)). Also, the production of CN(X2Z’(u= 0)) via dissociative attachment of low energy electrons by HCN is a distinct possibility. Although the maximum efficiency of our device (-0.1%) is low, the probable pumping mechanism suggests that a more efficient device is possible by suitably controlling the electron energy. The quantum efficiency of the laser is given by: quantum
270
efficiency =
h v (laser) electron energy
’
(3)
The authors acknowledge the technical assistance of H.R. Owen and J. Emerson, and the loan of equipment by W.H. Steier.
References 111J.C. Boden and B.A. Thrush, Proc. Roy. Sot. A305 (1968) 107.
PI D.D. Davis and H. Okabe, J. Chem. Phys. 49 (1968) 5526. [31 N. Basco, J.E. Nicholas, R.G.W. Norrish and W.H.J. Vickers, Proc. Roy. Sot. A272 (1963) 147. 141 W.M. Jackson, J. Chem. Phys. 61 (1974) 4177. 151 A. Mele and H. Okabe, J. Chem. Phys. 51 (1969) 4798. VI V.I. Korol and S.M. Kishko, Opt. Spectrosc. 33 (1972) 205. [71 I. Tokue, T. Urisu and K. Kuchitsu, J. Photochem. 3 (1974/ 75) 273. PI V.I. Korol and S.M. Kishko, Opt. Spectrosc. 38 (1975) 486. PI M.A. Pollack, Appl. Phys. Lett. 9 (1966) 230. [lOI G. West and M. Berry, J. Chem. Phys. 61 (1974) 4700. [Ill H.P. Broida, Appl. Opt. Suppl. on Chemical Lasers (1965) 105.