- Email: [email protected]
Switching of the two competing amplified spontaneous emission channels in NO
6 November 1998
Chemical Physics Letters 296 Ž1998. 384–390
Switching of the two competing amplified spontaneous emission channels in NO Yoshihiro Ogi, Koichi Tsukiyama
)
Department of Chemistry, Faculty of Science, Science UniÕersity of Tokyo, Kagurazaka, Shinjuku, Tokyo 162-8601, Japan Received 24 July 1998
Abstract Switching of the two competing amplified spontaneous emission ŽASE. channels is reported for the first time. The principle is based upon a characteristic of the ASE process that amplification occurs above a certain threshold of the population inversion density. We demonstrated that one of the two ASE channels could be suppressed almost completely by pre-populating the corresponding lower level. Enhancement of the other ASE channel implies the possibility of manipulating population transfer in molecular system. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction It has been commonly considered that spontaneous emission and reactive processes Žpredissociation, autoionization, etc.. generally dominate de-excitation from the laser-excited levels of the isolated molecules. However, recent extensive experimental investigations have shown that amplified spontaneous emission ŽASE., also termed stimulated emission ŽSE., might provide an additional relaxation pathway in molecular systems w1–7x. For example, Westblom et al. w3x reported two-photon driven ASE in the B1 Sq state of CO and compared characteristics of ASE with those of laser induced fluorescence ŽLIF.. More recently, our group w4–7x observed cascading ASE decay processes from the predissociative Rydberg states of NO, which indi-
)
Corresponding author. E-mail: [email protected]
cates the importance of ASE as a third radiative decay process in addition to fluorescence and phosphorescence. ASE is highly directional radiation emitted in an extended medium with distributed population inversion of atoms and molecules in absence of cavity mirrors. ASE processes can occur only above a certain threshold of the population inversion density determined from the molecular parameters such as radiative lifetimes and transition frequencies w8x. Accordingly, the productionrdestruction of the inverted population may result in ONrOFF of ASE, and thus control of the population transfer between the relevant excited states. The basic concept of the switching procedure is illustrated in Fig. 1. First, several rotational levels in the intermediate electronic state,
0009-2614r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 1 0 4 4 - 6
Y. Ogi, K. Tsukiyamar Chemical Physics Letters 296 (1998) 384–390
Fig. 1. Energy level diagram for the switching of amplified spontaneous emission.
that the ASE transition occurs from
2. Experiment The experimental arrangement is a modified version of that described in our previous paper w7x and only a brief statement is given below. A Nd:YAG laser ŽQuanta Ray, DCR-2A. pumped two dye lasers ŽQuanta Ray, PDL-2; hereafter referred to 1 and 3. simultaneously. The frequency of the dye laser 1 is doubled by a BBO crystal. The second harmonic Ž v 1 . around 226 nm excites the NO molecule from the X 2 P 3r2 Ž Õx s 0. state to the 3ssA 2 Sq Ž ÕA s 0.
385
state. The check of the resonant condition was performed by monitoring unresolved UV fluorescence from the A 2 Sq state at a cell 1. The visible output near 600 nm Ž v 3 . from the dye laser 3 excites the 4ss E 2 Sq Ž Õ E s 0. state from the laser-preared rotational levels in the A 2 Sq state, whereas the visible radiation near 480 nm Ž v 2 . from a dye laser 2 ŽContinuum, ND6000. pumped by a YAG laser ŽContinuum, Surelite-I. populates the 4ps M 2 Sq Ž Õ M s 0. state. Three laser beams are collineally combined with dichroic mirrors and directed into a stainless steel cell 2 Ž L s 10 cm. filled with a few Torr of NO. The ASE propagating in the same direction as the laser beams, after being separated from the laser radiation by filters and focused with a lens, is dispersed with a 27.5 cm monochromator ŽActon Research, SP402.. A Ge plate, a CaF2 lens Ž f s 200 mm., and a PbS ŽHamamatsu, P2680. or a PbSe detector ŽHamamatsu, P2682. are employed for the observation of the M ™ 3dp HX 2 Pq Ž; 5.2 mm. and M ™ E Ž; 2.6 mm. bands. As for the detection of the E ™ 3ps D 2 Sq Ž; 1.32 mm. and D ™ A Ž; 1.10 mm. bands, two color filters ŽHOYA, L38 and R80., a quartz lens Ž f s 120 mm., and a Ge detector ŽHamamatsu, B 1720-02. are used. The delay between the two YAG lasers is controlled by a digital delayrpulse generator ŽStanford Research Systems, DG535.. The pulse durations of v 1 , v 2 and v 3 are ; 7, ; 5 and ; 7 ns ŽFWHM., respectively. Under normal operations, three laser pulses are made coincident in time. The maximum pulse energies of v 1 , v 2 and v 3 radiation, which are varied by the use of neutral density filters, are estimated to be about 200, 20 and 200 mJrpulse, respectively. The beam diameter of v 3 is set much wider than those of v 1 and v 2 , which ensures that the special distribution Žconsidered as a slender column. of the M 2 Sq state is surrounded by that of the E 2 Sq state.
3. Results and discussion 3.1. Destruction of population inÕersion by collisional relaxation Before describing the laser-based method, we present a simpler scheme of the ASE switching in
386
Y. Ogi, K. Tsukiyamar Chemical Physics Letters 296 (1998) 384–390
which collisional relaxation in the intermediate manifold destroys population inversion. Rotational energy transfer in the A 2 Sq Ž ÕA s 0. state of NO with He and Ar gases was described in detail by Imajo et
al. w9x. In the present study, N2 was employed as a foreign gas which induces collisional quenching. In this scheme, only two independent laser outputs are required. A single rotational level in the E 2 Sq state
Fig. 2. Ža. Schematic energy level diagram Žnot scaled. of the cascading ASE decay processes from the E 2 Sq Ž Õ E s 0, NE s 4. level. Žb. Foreign gas pressure effect on the dispersed ASE spectra for the E 2 Sq™ D 2 Sq and D 2 Sq™ A 2 Sq bands. NO pressure: ; 1 Torr. The actual energies Žcmy1 . of the relevant electronic states: A 2 Sq Ž ÕA s 0, NA s 2. s 44210.96, D 2 Sq Ž Õ D s 0, ND s 3. s 53315.09, E 2 Sq Ž Õ E s 0, NE s 4. s 60902.35.
Y. Ogi, K. Tsukiyamar Chemical Physics Letters 296 (1998) 384–390
is selectively excited by optical–optical double resonance via the A 2 Sq state Žas shown in Fig. 2a.. The ASE de-excitation process from the E 2 Sq state has been analyzed in detail w5x; the initial decay is E ™ D around 1.32 mm followed by D ™ A around 1.10 mm. Fig. 2b shows the N2 pressure effect on the intensity pattern of these two ASE systems. When only 1 Torr of NO is in the cell, the intensity of the D ™ A transition is approximately one-third of that of E ™ D and all allowed emission lines Žindicated by downward arrows in Fig. 2a. appear. As the N2 pressure increases, the D ™ A band rapidly becomes weaker. This is obviously due to the reduction of the population inversion density induced by collisional relaxation in the A 2 Sq manifold. The reason of the sudden disappearance of the RŽ4. emission line is not clear. On the other hand, the E ™ D band is not so sensitive to the N2 pressure. The slower diminution of its intensity would be understandable as follows. Ž1. The laser prepared level in the A 2 Sq state is quenched by collision within the laser pulse duration. Consequently, the number of molecules brought up to the E 2 Sq state are reduced. Ž2. Under the absence of ASE from the D 2 Sq state, the main relaxation pathways are collisional quenching and spontaneous fluorescence to the A 2 Sq and X 2 P states. The rate of population damping by those two de-excitation paths is not as fast as the stimulated process. Thus the lack of the ASE channel leads to the longer trapping time in the D 2 Sq state, which in turn causes to hinder the E ™ D ASE channel. 3.2. Destruction of population inÕersion by laser excitation Fig. 3a shows a schematic diagram of the energy levels of NO illustrating the laser induced switching of ASE transitions. The energetics of NO require a somewhat more complicated excitation scheme than Fig. 3. Ža. Schematic energy level diagram Žnot scaled. of the cascading ASE decay processes from the M 2 Sq Ž Õ M s 0, NM s 2. level. Frequency of v 3 is tuned to pump the E 2 Sq Ž Õ E s 0, NE s 3. level. Žb. Dispersed ASE spectra corresponding to the M 2 Sq ™ E2 Sq band. Upper trace: v 3 OFF. Lower trace: v 3 ON. NO pressure: ; 2 Torr. The actual energies Žcmy1 . of the relevant electronic states: A 2 Sq Ž ÕA s 0, NA s 3. s 44222.88, D 2 Sq Ž Õ D s 0, ND s 0. s 53291.21, E 2 Sq Ž Õ E s 0, NE s 1. s 60866.75, M 2 Sq Ž Õ M s 0, NM s 2. s64671.3.
387
that of Fig. 1, but the essential arguments are the same. In particular, the first laser pumps a fraction of the ground state molecules into the A 2 Sq Ž ÕA s 0. state. Here it is of great importance that at least two rotational levels having different parities Žq and y. are populated simultaneously. In the present case the
Y. Ogi, K. Tsukiyamar Chemical Physics Letters 296 (1998) 384–390
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frequency v 1 is fixed on the band head of the P22 q Q 12 branches; the relatively broad spectral band width of the laser output Ž v 1 . causes the simultaneous excitation of both Fl and F2 components of A 2 Sq Ž ÕA s 0, NA s 0–6.. The second laser then selectively excites the M 2 Sq Ž Õ M s 0, NM s 2. level. It is known that the major ASE channel in the M 2 Sq state is the transition down to the E 2 Sq state around 2.6 mm w4,6x. The similar transition intensities of the R and P lines carry a nearly equal population down to NE s 1 and 3 levels, respectively. The third laser pumps one of the rotational levels combined by the ASE de-excitation from the A 2 Sq state; for instance, excitation by the PŽ4. line populates the NE s 3 level. Fig. 3b shows the dispersed ASE spectra belonging to the M ™ E transition; the upper and lower panels correspond to OFF and ON of the third laser, respectively. Each spectrum is plotted with the same scale on the vertical axis. As expected, the PŽ3. emission line disappears almost completely upon the introduction of the third laser. Importantly, the intensity of the RŽ1. emission line increases correspondingly and the enhancement seems to compensate the intensity drop of the PŽ3. line. Fig. 4b is good evidence to support that the population flow between the two competing optical paths can be manipulated by hindering the production of the population inversion. The critical number density, n c , for ASE amplification is written as w8x nc s
8p D n t L l2f
,
where D n is the width of the Doppler broadened transition, t the lifetime of the upper level, L the length of a column, l the emission wavelength, and f the branching ratio. From this equation, n c reduces to half when one of the two equivalent ASE channels is forbidden intentionally. Consequently, the ratio of the actual inversion density to n c is doubled. The gain for amplification is exponential near the threshold, but the region for the exponential growth is very narrow. Under the linear dependent regime, ASE intensity is also doubled, which complies with the spectrum shown in Fig. 3b. It should be mentioned that the complete switching was possible under a certain experimental condition. The spectra in Fig. 3b were taken with ; 200
mJrpulse of third pumping light Ž v 3 .. If much higher intensity Ž; 2 mJrpulse. was used, the PŽ3. line vanished completely and the RŽ1. line became weaker. This may be explainable qualitatively as follows. As shown in Fig. 3a, the strong excitation of NE s 3 by v 3 induces the intense ASE from NE s 3 down to ND s 2 ŽRŽ2. line in the E ™ D band.. The population in ND s 2 remains for more than 20 ns, the radiative lifetime of the D 2 Sq state w10x, because NA s 0–6 levels are excited simultaneously by v 1 and ASE transitions from D to A are forbidden. The population trapping in ND s 2 disturbs the stimulated population transfer from NE s 1 to ND s 2. The obstruction of the rapid population damping in NE s 1 reduces the population inversion density between NM s 2 and NE s 1, resulting in the diminution of the RŽ1. line. Another scheme, sketched in Fig. 4a, is analogous to Fig. 3a; the only difference is the rotational quantum number in the M 2 Sq state. When the NM s 0 level is selectively pumped by v 2 , the P emission line is the only possible de-excitation path in the M ™ E band. As a minor ASE channel, the M ™ HX 2 Pq system was observed around 5.2 mm w6x. In this case, two competing optical paths to be controlled belong to the different inter-Rydberg transitions. Adjusting carefully the intensity of the second pumping laser, we can construct an environment in which only the stronger transition is observable as shown in the upper trace in Fig. 4b. These conditions are achievable when the oscillator strengths of the relevant transitions differ considerably. A similar example was found recently in the E state w11x. When the laser intensity is moderate, the E ™ D system is much stronger than the E ™ C system and the latter is almost undetectable. As the higher laser power is applied, the E ™ C system becomes stronger. The mixing of the 4ss character with the 3ds in the E 2 Sq state yields the large intensity difference between these two inter-Rydberg transitions; destructive interference of the 4ss–3pp and 3ds–3dp transition dipoles causes the reduction of the E ™ C transition intensity w12x. As seen in the lower panel of Fig. 4b, when v 3 is on, the M ™ E ASE line reduces its intensity and the M ™ HX line appears, which indicates the feasibility of switching of the population transfer between two different electronic transitions.
Y. Ogi, K. Tsukiyamar Chemical Physics Letters 296 (1998) 384–390
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Fig. 4. Ža. Schematic energy level diagram Žnot scaled. showing two ASE relaxation channels from the M 2Sq Ž Õ M s 0, NM s 0. level. v 3 frequency is tuned to pump the E 2 Sq Ž Õ E s 0, NE s 1. level. Žb. Dispersed ASE spectra corresponding to the M 2 Sq™ E 2 Sq and X M 2 Sq™ H 2 Pq bands. Upper trace: v 3 OFF. Lower trace: v 3 ON. A wider monochromator slit width causes broader line profiles than those in Figs. 2 and 3. NO pressure: ; 5 Torr. The actual energies Žcmy1 . of the relevant electronic states: A 2 Sq Ž ÕA s 0, NA s 1. s X 44203.02, E 2 Sq Ž Õ E s 0, NE s 1. s 60866.75, H 2 Pq Ž Õ H X s 0, J H X s 1.5, F1 . s 62724.7, M 2 Sq Ž Õ M s 0, NM s 0, J M s 0.5. s 64659.0.
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Y. Ogi, K. Tsukiyamar Chemical Physics Letters 296 (1998) 384–390
In conclusion, we demonstrated the switching of the two competing ASE decay channels by manipulating inverted population. Two methods have been presented for the destruction of inverted population. One utilizes rotational redistribution by collisions, which is simpler but no quantum state selectivity is anticipated. The other takes advantage of the laser excitation in which the lower level of the ASE transition is intentionally pre-populated. We succeeded in changing the branching ratio Ži. between P and R branches belonging to the same electronic system and Žii. between two different electronic transitions. If this method is applied to the level in which the ASE radiative channel is competing with the reactive channels such as predissociation and autoionization, it is quite probable that switching of ASE leads to control of the quantum yields for reactive channels producing photofragments and molecular ions.
Acknowledgements This work is partly supported by a Grant-in-Aid ŽNo. 10640497. from the Ministry of Education,
Science, Sports, and Culture, Japan. The authors are pleased to thank Miss M. Takahashi, Mr. S. Sato, and Mr. K. Hattori for their help in the experiment.
References w1x U. Czarnetzki, H.F. Dobele, Phys. Rev. A 44 Ž1991. 7530. ¨ w2x J.W. Glessner, S.J. Davis, J. Appl. Phys. 73 Ž1993. 2672. w3x U. Westblom, S. Agrup, M. Alden, H.M. Herz, J.E.M. Goldsmith, Appl. Phys. B: Photophys. Laser Chem. 50 Ž1990. 487. w4x J. Ishii, K. Uehara, K. Tsukiyama, J. Chem. Phys. 104 Ž1996. 499. w5x K. Tsukiyama, J. Phys. Chem. A 101 Ž1997. 360. w6x Y. Ogi, A. Sugita, H. Suzuki, Y. Tanaka, K. Tsukiyama, J. Chem. Phys. 108 Ž1998. 900. w7x A. Sugita, M. Ikeda, K. Tsukiyama, J. Chem. Phys. 109 Ž1998. 3386. w8x G.I. Peters, L. Allen, J. Phys. A 4 Ž1971. 238. w9x T. Imajo, K. Shibuya, K. Obi, Chem. Phys. Lett. 137 Ž1987. 139. w10x K. Tsukiyama, T. Munakata, M. Tsukakoshi, T. Kasuya, Chem. Phys. 121 Ž1988. 55. w11x Y. Ogi, K. Tsukiyama, Unpublished results. w12x R. Gallusser, K. Dressler, J. Appl. Math. Phys. ŽZAMP. 22 Ž1971. 792.
Chemical Physics Letters 296 Ž1998. 384–390
Switching of the two competing amplified spontaneous emission channels in NO Yoshihiro Ogi, Koichi Tsukiyama
)
Department of Chemistry, Faculty of Science, Science UniÕersity of Tokyo, Kagurazaka, Shinjuku, Tokyo 162-8601, Japan Received 24 July 1998
Abstract Switching of the two competing amplified spontaneous emission ŽASE. channels is reported for the first time. The principle is based upon a characteristic of the ASE process that amplification occurs above a certain threshold of the population inversion density. We demonstrated that one of the two ASE channels could be suppressed almost completely by pre-populating the corresponding lower level. Enhancement of the other ASE channel implies the possibility of manipulating population transfer in molecular system. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction It has been commonly considered that spontaneous emission and reactive processes Žpredissociation, autoionization, etc.. generally dominate de-excitation from the laser-excited levels of the isolated molecules. However, recent extensive experimental investigations have shown that amplified spontaneous emission ŽASE., also termed stimulated emission ŽSE., might provide an additional relaxation pathway in molecular systems w1–7x. For example, Westblom et al. w3x reported two-photon driven ASE in the B1 Sq state of CO and compared characteristics of ASE with those of laser induced fluorescence ŽLIF.. More recently, our group w4–7x observed cascading ASE decay processes from the predissociative Rydberg states of NO, which indi-
)
Corresponding author. E-mail: [email protected]
cates the importance of ASE as a third radiative decay process in addition to fluorescence and phosphorescence. ASE is highly directional radiation emitted in an extended medium with distributed population inversion of atoms and molecules in absence of cavity mirrors. ASE processes can occur only above a certain threshold of the population inversion density determined from the molecular parameters such as radiative lifetimes and transition frequencies w8x. Accordingly, the productionrdestruction of the inverted population may result in ONrOFF of ASE, and thus control of the population transfer between the relevant excited states. The basic concept of the switching procedure is illustrated in Fig. 1. First, several rotational levels in the intermediate electronic state,
0009-2614r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 1 0 4 4 - 6
Y. Ogi, K. Tsukiyamar Chemical Physics Letters 296 (1998) 384–390
Fig. 1. Energy level diagram for the switching of amplified spontaneous emission.
that the ASE transition occurs from
2. Experiment The experimental arrangement is a modified version of that described in our previous paper w7x and only a brief statement is given below. A Nd:YAG laser ŽQuanta Ray, DCR-2A. pumped two dye lasers ŽQuanta Ray, PDL-2; hereafter referred to 1 and 3. simultaneously. The frequency of the dye laser 1 is doubled by a BBO crystal. The second harmonic Ž v 1 . around 226 nm excites the NO molecule from the X 2 P 3r2 Ž Õx s 0. state to the 3ssA 2 Sq Ž ÕA s 0.
385
state. The check of the resonant condition was performed by monitoring unresolved UV fluorescence from the A 2 Sq state at a cell 1. The visible output near 600 nm Ž v 3 . from the dye laser 3 excites the 4ss E 2 Sq Ž Õ E s 0. state from the laser-preared rotational levels in the A 2 Sq state, whereas the visible radiation near 480 nm Ž v 2 . from a dye laser 2 ŽContinuum, ND6000. pumped by a YAG laser ŽContinuum, Surelite-I. populates the 4ps M 2 Sq Ž Õ M s 0. state. Three laser beams are collineally combined with dichroic mirrors and directed into a stainless steel cell 2 Ž L s 10 cm. filled with a few Torr of NO. The ASE propagating in the same direction as the laser beams, after being separated from the laser radiation by filters and focused with a lens, is dispersed with a 27.5 cm monochromator ŽActon Research, SP402.. A Ge plate, a CaF2 lens Ž f s 200 mm., and a PbS ŽHamamatsu, P2680. or a PbSe detector ŽHamamatsu, P2682. are employed for the observation of the M ™ 3dp HX 2 Pq Ž; 5.2 mm. and M ™ E Ž; 2.6 mm. bands. As for the detection of the E ™ 3ps D 2 Sq Ž; 1.32 mm. and D ™ A Ž; 1.10 mm. bands, two color filters ŽHOYA, L38 and R80., a quartz lens Ž f s 120 mm., and a Ge detector ŽHamamatsu, B 1720-02. are used. The delay between the two YAG lasers is controlled by a digital delayrpulse generator ŽStanford Research Systems, DG535.. The pulse durations of v 1 , v 2 and v 3 are ; 7, ; 5 and ; 7 ns ŽFWHM., respectively. Under normal operations, three laser pulses are made coincident in time. The maximum pulse energies of v 1 , v 2 and v 3 radiation, which are varied by the use of neutral density filters, are estimated to be about 200, 20 and 200 mJrpulse, respectively. The beam diameter of v 3 is set much wider than those of v 1 and v 2 , which ensures that the special distribution Žconsidered as a slender column. of the M 2 Sq state is surrounded by that of the E 2 Sq state.
3. Results and discussion 3.1. Destruction of population inÕersion by collisional relaxation Before describing the laser-based method, we present a simpler scheme of the ASE switching in
386
Y. Ogi, K. Tsukiyamar Chemical Physics Letters 296 (1998) 384–390
which collisional relaxation in the intermediate manifold destroys population inversion. Rotational energy transfer in the A 2 Sq Ž ÕA s 0. state of NO with He and Ar gases was described in detail by Imajo et
al. w9x. In the present study, N2 was employed as a foreign gas which induces collisional quenching. In this scheme, only two independent laser outputs are required. A single rotational level in the E 2 Sq state
Fig. 2. Ža. Schematic energy level diagram Žnot scaled. of the cascading ASE decay processes from the E 2 Sq Ž Õ E s 0, NE s 4. level. Žb. Foreign gas pressure effect on the dispersed ASE spectra for the E 2 Sq™ D 2 Sq and D 2 Sq™ A 2 Sq bands. NO pressure: ; 1 Torr. The actual energies Žcmy1 . of the relevant electronic states: A 2 Sq Ž ÕA s 0, NA s 2. s 44210.96, D 2 Sq Ž Õ D s 0, ND s 3. s 53315.09, E 2 Sq Ž Õ E s 0, NE s 4. s 60902.35.
Y. Ogi, K. Tsukiyamar Chemical Physics Letters 296 (1998) 384–390
is selectively excited by optical–optical double resonance via the A 2 Sq state Žas shown in Fig. 2a.. The ASE de-excitation process from the E 2 Sq state has been analyzed in detail w5x; the initial decay is E ™ D around 1.32 mm followed by D ™ A around 1.10 mm. Fig. 2b shows the N2 pressure effect on the intensity pattern of these two ASE systems. When only 1 Torr of NO is in the cell, the intensity of the D ™ A transition is approximately one-third of that of E ™ D and all allowed emission lines Žindicated by downward arrows in Fig. 2a. appear. As the N2 pressure increases, the D ™ A band rapidly becomes weaker. This is obviously due to the reduction of the population inversion density induced by collisional relaxation in the A 2 Sq manifold. The reason of the sudden disappearance of the RŽ4. emission line is not clear. On the other hand, the E ™ D band is not so sensitive to the N2 pressure. The slower diminution of its intensity would be understandable as follows. Ž1. The laser prepared level in the A 2 Sq state is quenched by collision within the laser pulse duration. Consequently, the number of molecules brought up to the E 2 Sq state are reduced. Ž2. Under the absence of ASE from the D 2 Sq state, the main relaxation pathways are collisional quenching and spontaneous fluorescence to the A 2 Sq and X 2 P states. The rate of population damping by those two de-excitation paths is not as fast as the stimulated process. Thus the lack of the ASE channel leads to the longer trapping time in the D 2 Sq state, which in turn causes to hinder the E ™ D ASE channel. 3.2. Destruction of population inÕersion by laser excitation Fig. 3a shows a schematic diagram of the energy levels of NO illustrating the laser induced switching of ASE transitions. The energetics of NO require a somewhat more complicated excitation scheme than Fig. 3. Ža. Schematic energy level diagram Žnot scaled. of the cascading ASE decay processes from the M 2 Sq Ž Õ M s 0, NM s 2. level. Frequency of v 3 is tuned to pump the E 2 Sq Ž Õ E s 0, NE s 3. level. Žb. Dispersed ASE spectra corresponding to the M 2 Sq ™ E2 Sq band. Upper trace: v 3 OFF. Lower trace: v 3 ON. NO pressure: ; 2 Torr. The actual energies Žcmy1 . of the relevant electronic states: A 2 Sq Ž ÕA s 0, NA s 3. s 44222.88, D 2 Sq Ž Õ D s 0, ND s 0. s 53291.21, E 2 Sq Ž Õ E s 0, NE s 1. s 60866.75, M 2 Sq Ž Õ M s 0, NM s 2. s64671.3.
387
that of Fig. 1, but the essential arguments are the same. In particular, the first laser pumps a fraction of the ground state molecules into the A 2 Sq Ž ÕA s 0. state. Here it is of great importance that at least two rotational levels having different parities Žq and y. are populated simultaneously. In the present case the
Y. Ogi, K. Tsukiyamar Chemical Physics Letters 296 (1998) 384–390
388
frequency v 1 is fixed on the band head of the P22 q Q 12 branches; the relatively broad spectral band width of the laser output Ž v 1 . causes the simultaneous excitation of both Fl and F2 components of A 2 Sq Ž ÕA s 0, NA s 0–6.. The second laser then selectively excites the M 2 Sq Ž Õ M s 0, NM s 2. level. It is known that the major ASE channel in the M 2 Sq state is the transition down to the E 2 Sq state around 2.6 mm w4,6x. The similar transition intensities of the R and P lines carry a nearly equal population down to NE s 1 and 3 levels, respectively. The third laser pumps one of the rotational levels combined by the ASE de-excitation from the A 2 Sq state; for instance, excitation by the PŽ4. line populates the NE s 3 level. Fig. 3b shows the dispersed ASE spectra belonging to the M ™ E transition; the upper and lower panels correspond to OFF and ON of the third laser, respectively. Each spectrum is plotted with the same scale on the vertical axis. As expected, the PŽ3. emission line disappears almost completely upon the introduction of the third laser. Importantly, the intensity of the RŽ1. emission line increases correspondingly and the enhancement seems to compensate the intensity drop of the PŽ3. line. Fig. 4b is good evidence to support that the population flow between the two competing optical paths can be manipulated by hindering the production of the population inversion. The critical number density, n c , for ASE amplification is written as w8x nc s
8p D n t L l2f
,
where D n is the width of the Doppler broadened transition, t the lifetime of the upper level, L the length of a column, l the emission wavelength, and f the branching ratio. From this equation, n c reduces to half when one of the two equivalent ASE channels is forbidden intentionally. Consequently, the ratio of the actual inversion density to n c is doubled. The gain for amplification is exponential near the threshold, but the region for the exponential growth is very narrow. Under the linear dependent regime, ASE intensity is also doubled, which complies with the spectrum shown in Fig. 3b. It should be mentioned that the complete switching was possible under a certain experimental condition. The spectra in Fig. 3b were taken with ; 200
mJrpulse of third pumping light Ž v 3 .. If much higher intensity Ž; 2 mJrpulse. was used, the PŽ3. line vanished completely and the RŽ1. line became weaker. This may be explainable qualitatively as follows. As shown in Fig. 3a, the strong excitation of NE s 3 by v 3 induces the intense ASE from NE s 3 down to ND s 2 ŽRŽ2. line in the E ™ D band.. The population in ND s 2 remains for more than 20 ns, the radiative lifetime of the D 2 Sq state w10x, because NA s 0–6 levels are excited simultaneously by v 1 and ASE transitions from D to A are forbidden. The population trapping in ND s 2 disturbs the stimulated population transfer from NE s 1 to ND s 2. The obstruction of the rapid population damping in NE s 1 reduces the population inversion density between NM s 2 and NE s 1, resulting in the diminution of the RŽ1. line. Another scheme, sketched in Fig. 4a, is analogous to Fig. 3a; the only difference is the rotational quantum number in the M 2 Sq state. When the NM s 0 level is selectively pumped by v 2 , the P emission line is the only possible de-excitation path in the M ™ E band. As a minor ASE channel, the M ™ HX 2 Pq system was observed around 5.2 mm w6x. In this case, two competing optical paths to be controlled belong to the different inter-Rydberg transitions. Adjusting carefully the intensity of the second pumping laser, we can construct an environment in which only the stronger transition is observable as shown in the upper trace in Fig. 4b. These conditions are achievable when the oscillator strengths of the relevant transitions differ considerably. A similar example was found recently in the E state w11x. When the laser intensity is moderate, the E ™ D system is much stronger than the E ™ C system and the latter is almost undetectable. As the higher laser power is applied, the E ™ C system becomes stronger. The mixing of the 4ss character with the 3ds in the E 2 Sq state yields the large intensity difference between these two inter-Rydberg transitions; destructive interference of the 4ss–3pp and 3ds–3dp transition dipoles causes the reduction of the E ™ C transition intensity w12x. As seen in the lower panel of Fig. 4b, when v 3 is on, the M ™ E ASE line reduces its intensity and the M ™ HX line appears, which indicates the feasibility of switching of the population transfer between two different electronic transitions.
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Fig. 4. Ža. Schematic energy level diagram Žnot scaled. showing two ASE relaxation channels from the M 2Sq Ž Õ M s 0, NM s 0. level. v 3 frequency is tuned to pump the E 2 Sq Ž Õ E s 0, NE s 1. level. Žb. Dispersed ASE spectra corresponding to the M 2 Sq™ E 2 Sq and X M 2 Sq™ H 2 Pq bands. Upper trace: v 3 OFF. Lower trace: v 3 ON. A wider monochromator slit width causes broader line profiles than those in Figs. 2 and 3. NO pressure: ; 5 Torr. The actual energies Žcmy1 . of the relevant electronic states: A 2 Sq Ž ÕA s 0, NA s 1. s X 44203.02, E 2 Sq Ž Õ E s 0, NE s 1. s 60866.75, H 2 Pq Ž Õ H X s 0, J H X s 1.5, F1 . s 62724.7, M 2 Sq Ž Õ M s 0, NM s 0, J M s 0.5. s 64659.0.
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In conclusion, we demonstrated the switching of the two competing ASE decay channels by manipulating inverted population. Two methods have been presented for the destruction of inverted population. One utilizes rotational redistribution by collisions, which is simpler but no quantum state selectivity is anticipated. The other takes advantage of the laser excitation in which the lower level of the ASE transition is intentionally pre-populated. We succeeded in changing the branching ratio Ži. between P and R branches belonging to the same electronic system and Žii. between two different electronic transitions. If this method is applied to the level in which the ASE radiative channel is competing with the reactive channels such as predissociation and autoionization, it is quite probable that switching of ASE leads to control of the quantum yields for reactive channels producing photofragments and molecular ions.
Acknowledgements This work is partly supported by a Grant-in-Aid ŽNo. 10640497. from the Ministry of Education,
Science, Sports, and Culture, Japan. The authors are pleased to thank Miss M. Takahashi, Mr. S. Sato, and Mr. K. Hattori for their help in the experiment.
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