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Observation of laser driven transitions to high rydberg states of He2
Volume 74, number 2
CHEhIICAL PHYSICS LETTERS
OBSERVATION
TO EIGEI RYDBERG STATES OF Her
OF LASER DRIVEN TRANSITIONS
R. PANOCK, R.R. FREEMAN, Bell Telephone
Laboratones.
1 September 1980
RI-L STORZ
Holmdel, New Jersey 07733. USA
and Terry A. MILLER Bell Telephone Laboratories,
hXurray Hdl, New Jersey 07974. US4
Received 19 July 1980
We report the exatation and detection of Rydberg states of He, with principal quantum numbers 5 25. The tmnsitions were driven from the metastable (2s~) a3 XL He, state, and the exated states were detected using ionization techniqu% Q branch trans~hons termmating on nps~3~ (u = 0) states up to n = 25 have been identifiedand wigned.
1. Introduction Although high Rydberg states of atoms have been studied extensively over the past decade [ 11, comparatively httle work has been done with highly excited states of molecular systems. Rydberg series have of course been observed for NO, N2, 02, CO [2], Hz [3], and recently in Na2 [4], but with the exception of He, to be discussed below, most of the work has been limited to relatively low principal quantum number (n) or resolution. This 1s due to the usual difficulty of decreased transition strength and reduced fluorescence yield with increasing n. Molecules have the added problem of much more complex structure m their Rydberg states than atoms, making the identification of such states more difficult. Moreover, molecular predissociation often causes molecular Rydberg series to be IU defined. We report here the initial results of a study of the Rydberg states of He, m which an ionization technique is used to detect laser driven transitions to highly excited np Rydberg states (ii excess of n = 25). The relative simplicity of the He, molecule suggests that the identification of Rydberg states is more tractable than for many other molecules. Likewise in He,, the Rydberg series does not appear to be strongly predissociated. However, because He, possesses no bound
ground state, it is not eanly studied. Most of the previous work on this molecule has been performed by classical spectroscopic techniques Iusingoptical emission from a discharge source [5]. This previous work is rather extensive for n 5 17 and has recently formed the basis of a multichannel quantum defect theory (MQDT) analysis of the P and R branches [6] of the np A 311_iand npa 3Zi states. (There have been other studies involving absorption [7] and molecular beams [S] , but these have been restricted to low-lying states.) The only previous laser excitation work on He2 [9] relied upon fluorescence detection after excitation and, as a consequence, did not involve excited states withn>15. We have recently described [lo] a technique for the laser excitation and subseiuent ionization detection of highly excited states of atoms. The initial state in these transitions is a metastable one formed by an electric discharge. This technique has several attributes which make it attractive for use in studying molecular Rydberg states. in contrast TVclassical, experimental, optical techniques or laser fluorescence schemes, thz sensitivity of detection of excited states does not decrease with increasing principal quantum number. Indeed the technique is almost ideally complementary to standard techniques that rely upon the detection of fluorescent photons. For states with low quantum 203
Volume
CHEMICAL.
74, number 2
yield for fluorescence because of competing ionization processes, our techmque 1s most sensrtive. The detection region IS field free (a necessary conditron for the study of highly excited states) yet it works well under conditions m which a relatively high pressure drscharge is requued to form the species of interest. In thrs letter we report the observation of a rich and comphcated laser excitation spectrum in He, between 2900 and 3050 A. The transrtions originate from the rotatronal levels of the b” = 0 or u” = 1 (2so)a 3Ct metastable state, the final states being the nprr 3fI- (Q branch transrtions), npn 311i or the rzpu 3 ZZQ (R and P branch transitrons) u’ = 0 and L”= 1 states. + he highest pnncipal quantum we have identified IS n = 25, although
problem appears observation.
number II which for higher tz the
to be one of identificatron
and not
2. Apparatus The apparatus used m this study has been described prevrously [lo] _ The He, molecules are formed in a dc electric discharge at a pressure of S-15 Torr. The gas flows downstream between two stainless steel plates. The maxunum number of He, molecules delivered to the mteraction region was hmited by the maxrmum pressure (= 15 Torr) or drscharge current (= 2 mA) we could employ before plasma osctiatrons from the discharge were picked up on the plates. Coherent light of bandwidth LIV= 0.15 cm-l and energy of O-l-O.2 mJ/pulse was obtamed by doubhng light from a Littman-style dye laser operating at 10 pps. When the laser hght was coincident with a possrble molecular transrtion, an ronization signal was detected by the plates. The ionization srgnal presumably arises from the productron of ions produced from highly excited Rydberg molecules which colhsionally, associatively, or autoronize and are swept out by the plates. When we performed a sundar experiment on atomic helium, under the same expenmental condttions, transitions to states wrth n = 40 were observed wrth no indication of sigmficant electnc field nuxmg.
3. Observations A portion 204
of our data, between
3003 and 3008 A
1 September 1980
PHYSICS LETTERS
is shown m fig. 1. The most readily identified resonance lines in fig. 1 are those of a Q branch transition, involvmg the essentially unperturbed levels of a llg state. Because He2 contains two identical zero spin nuclei, Q branch transitions for even rotational levels are non-existent. Any triplet splitting is below the experimental resolution. Thus each group of AN = 0 excrtations terminatmg on a grven electronic excited state forms an easdy assigned Rydberg series. We have been able to tentatively identify members of thrs series up to tz = 25 using a standard Born-Oppenheimer analysis with constants extrapolated from the lower lying electronic states. We fiid that thrs method predicts all observed Q transitions up to IZ = 25 to wrtlun f 0.5 cm-t _ To our surpnse, the u’ = 1 * u” = 1 transitions were nearly as strong as the u’ = 0 f u” = 0 transitions, for n 2 20. (Jndeed, the Q branch lines m fig. 1 are u = 1 + 1.) We have not been able to determine whether thrs is due to our source producing a non-equlbrium drstnbution of vrbrationally excited metastable states, or to possible increases in our detectron efficiency with vrbratronal excitation. Nonetheless, we have observed the transltrons @prr) 3JJg(u = 1) + (2su)a 32i (u”=l)forlO
0
6007
5
33281 7
~A&‘)
60;7
utcm-‘1
33226 4
FIN. 1. IoruzaUon signal versus exaration
5
wavelength. ha&i) IS the wavelength of the undoubled dye laser, “(cm-‘) is the correspondmg transltion frequency. The Q branch m&cated isforthe(11p7r)u3t$ (u’=I)+(2sa)a32~ (u”=l)trans~tion (the stars mdcating nbrationally excited states). Other P and R branch transitions are observed as weU as, of yet, umdenticd hnes
CHEhiICAL PIiYSICS LETTERS
Volume 74. number 2
sitions are at wavelengths which are obscured by the strong atomic helium 2 3S-5 3P transition. In the wavelength range of our investigation there are two possible 523 = 1 off-diagonal wbrational transitions. We were able to observe the (Splr) s 3lTt (u’ = 1) f (2s~) a 3X: (v” = 1) transitions w&rout diffkx&y around h = 2925 A. However, we were unable to observe the (npn) r 3ZZg (u’ = 1) + (2s~) a 3 Xi (v” = 0) transitions around X = 2975 A. The positions of the P and R branch np transrtions are not so easily predicted and assigned as the Q branch. This is due to the strong perturbations between the IZ+ and lI+ states which render a standard spectroscopic analysis completely inadequate_ The P and R branch lines begin to overlap the Q branch series above n = 11, and by n = 16 the Q branch spectrum becomes quote congested by P and R branch lines. In fig. 2 a portion of the recorded spectrum between 2950 and 2955 A is shown where the Q branch spectrum of n = 16 is intermixed with P and R branch lures from levels with 12 in the range 16-2 1. The only tractable method of handling the P and R branch lures is by a MQDT analysis, the assignments shown III fig 2 were derived using a preliminary MQDT analysis based upon the constants of Ginter and Ginter [6 1. A much more thorough MQDT analysis, necessary for the predictron and assignment of the very high n states, IS in progress and wdl be reported at a later date.
i
58534
341230
1 September
L980
i
!ias4 34064B
Fig. 3 Ionization signal versus wavelength. Assrgnments are tentative and serve to indicate the predicted positions for the venous Q branch transitions.
Fig. 3 shows a portron of the observed spectra in the region of n = 25. Many of the transitions observed in fig. 3 are to states that are energetically able to autoionize- For n 2 9 dl excited vibrational(u Z 1) states lie above the u = 0, N = 0 level of the X 2xz state of Her. Many of the u = 0 Rydberg states observed also lie above the u = 0, N = 0 2Ei state of He; due to their rotational energy. Of the states capable of autoio~atio~, some have transitions that appear no broader than inskumentaf, others however appear broadened. Very interestingiy there appears to be Ln some places (see fig. 3) a structured continuum perhaps attributable to Beutler-Fano absorption. When a complete identificationand MQDT prediction of the line positions is compIeted, we should be abk to probe these processesin considerabledetail.
4. Conclusion
59&l
338818
l&ii)
v(crnd)
m*t 338245
Fzg 2. Iomzation signal Y+ISUSextxtahon wavelength over the regon of the Q branch transxtlons (16pd3$ {u’ = 0) +- (2s~)
a3Zt
We have describeda techniqueuseful for the observation of molecular Rydberg states- This technique in two complements optical detection m 7 -t ways. We can observe states with long radiative lifetunes (high n) that cannot bz observedin fluorescence, and we can observe states with datidy shoa kmiation lifetimes due to co&sional or autoionizationprocessesthat againwould not be observedin fluores-
Volume
CHEMICAL
74, number 2
cence. We have observed states m He2 up to II 2: 25 and have identified many new transitrons. Further analysis using an expanded MQDT treatment should allow a full identification of the observed structure as well as a basis for mvestigations of the stabrlity of various levels against autoionization.
Acknowledgement We thank M. Cinter and D. Gmter for conversations and a preprint on optical emission studres of He, Rydberg states and for a careful reading of the manuscript, and V.E. Bondybey for helpful suggestions.
References [ 1]
h1.L Zrmmerman. hl G. Lrttman, bl Kash and D Kleppner, Ph) s. Rev- AZ0 (1980) 215 1, and K P. Huber and G. Herzberg,
molecules (Van Nostrand,
‘06
LEl-l-ERS
refer-
Constants of dlatomlc Prmceton. 1979).
1
September 1980
[3] P.M Debmer and W.A. Chupka, J. Chem Phys. 65 (1976) 2243.66 (1977) 1972. G Herzberg and Ch. Jungen, J. hfol. Spectry. 41 (1972) 42.5. [4] N.W Carlson, private commumcatlon [S] hI.L Gmter, J. Chem. Phys. 42 (1965) 561. M L Gutter and D S Cmter, J. Chem Phys. 48 (1968) 2284; M.L. Gmter and R. Batteno. J Chem. Phys 52 (1970) 4469; hf L. Gmter, J. Chem. Phys 45 (1966) 248, J. Mol. Spectry. 17 (1965) 224, J. Chem. Phys. 18 (1965) 321, F B. Orth and M L Gmter, J. Chem Phys 61 (1976) 282,64 (1977) 223, F B. Orth, CM. Brown and M.L. Gmter. J. Chem. Phys 69 (1978) 53. [6] D S. Gmter and hI L. Gmter, J. Mol. Spectry.. to be pubhhed [7] A B. Callear and R E M. Hedges, Trans Faraday Sot 66 (1970) 2921. [S] W Lchren. hl_V. M cCusher and T-IVienna, J. Chem. F’hys.
ences therem. [2]
PHYSICS
61 (1974) 2200,
T. mema, J. Chem Phls 62 (1975) 2925. [9] T-k howler and V_E Bondybey, J. hfoL Spectry. 78 (1979) 120. [IO] R Panock, R.R. Freeman, J C. White and R-H. Storz, Opt Letters 5 (1980) 160.
CHEhIICAL PHYSICS LETTERS
OBSERVATION
TO EIGEI RYDBERG STATES OF Her
OF LASER DRIVEN TRANSITIONS
R. PANOCK, R.R. FREEMAN, Bell Telephone
Laboratones.
1 September 1980
RI-L STORZ
Holmdel, New Jersey 07733. USA
and Terry A. MILLER Bell Telephone Laboratories,
hXurray Hdl, New Jersey 07974. US4
Received 19 July 1980
We report the exatation and detection of Rydberg states of He, with principal quantum numbers 5 25. The tmnsitions were driven from the metastable (2s~) a3 XL He, state, and the exated states were detected using ionization techniqu% Q branch trans~hons termmating on nps~3~ (u = 0) states up to n = 25 have been identifiedand wigned.
1. Introduction Although high Rydberg states of atoms have been studied extensively over the past decade [ 11, comparatively httle work has been done with highly excited states of molecular systems. Rydberg series have of course been observed for NO, N2, 02, CO [2], Hz [3], and recently in Na2 [4], but with the exception of He, to be discussed below, most of the work has been limited to relatively low principal quantum number (n) or resolution. This 1s due to the usual difficulty of decreased transition strength and reduced fluorescence yield with increasing n. Molecules have the added problem of much more complex structure m their Rydberg states than atoms, making the identification of such states more difficult. Moreover, molecular predissociation often causes molecular Rydberg series to be IU defined. We report here the initial results of a study of the Rydberg states of He, m which an ionization technique is used to detect laser driven transitions to highly excited np Rydberg states (ii excess of n = 25). The relative simplicity of the He, molecule suggests that the identification of Rydberg states is more tractable than for many other molecules. Likewise in He,, the Rydberg series does not appear to be strongly predissociated. However, because He, possesses no bound
ground state, it is not eanly studied. Most of the previous work on this molecule has been performed by classical spectroscopic techniques Iusingoptical emission from a discharge source [5]. This previous work is rather extensive for n 5 17 and has recently formed the basis of a multichannel quantum defect theory (MQDT) analysis of the P and R branches [6] of the np A 311_iand npa 3Zi states. (There have been other studies involving absorption [7] and molecular beams [S] , but these have been restricted to low-lying states.) The only previous laser excitation work on He2 [9] relied upon fluorescence detection after excitation and, as a consequence, did not involve excited states withn>15. We have recently described [lo] a technique for the laser excitation and subseiuent ionization detection of highly excited states of atoms. The initial state in these transitions is a metastable one formed by an electric discharge. This technique has several attributes which make it attractive for use in studying molecular Rydberg states. in contrast TVclassical, experimental, optical techniques or laser fluorescence schemes, thz sensitivity of detection of excited states does not decrease with increasing principal quantum number. Indeed the technique is almost ideally complementary to standard techniques that rely upon the detection of fluorescent photons. For states with low quantum 203
Volume
CHEMICAL.
74, number 2
yield for fluorescence because of competing ionization processes, our techmque 1s most sensrtive. The detection region IS field free (a necessary conditron for the study of highly excited states) yet it works well under conditions m which a relatively high pressure drscharge is requued to form the species of interest. In thrs letter we report the observation of a rich and comphcated laser excitation spectrum in He, between 2900 and 3050 A. The transrtions originate from the rotatronal levels of the b” = 0 or u” = 1 (2so)a 3Ct metastable state, the final states being the nprr 3fI- (Q branch transrtions), npn 311i or the rzpu 3 ZZQ (R and P branch transitrons) u’ = 0 and L”= 1 states. + he highest pnncipal quantum we have identified IS n = 25, although
problem appears observation.
number II which for higher tz the
to be one of identificatron
and not
2. Apparatus The apparatus used m this study has been described prevrously [lo] _ The He, molecules are formed in a dc electric discharge at a pressure of S-15 Torr. The gas flows downstream between two stainless steel plates. The maxunum number of He, molecules delivered to the mteraction region was hmited by the maxrmum pressure (= 15 Torr) or drscharge current (= 2 mA) we could employ before plasma osctiatrons from the discharge were picked up on the plates. Coherent light of bandwidth LIV= 0.15 cm-l and energy of O-l-O.2 mJ/pulse was obtamed by doubhng light from a Littman-style dye laser operating at 10 pps. When the laser hght was coincident with a possrble molecular transrtion, an ronization signal was detected by the plates. The ionization srgnal presumably arises from the productron of ions produced from highly excited Rydberg molecules which colhsionally, associatively, or autoronize and are swept out by the plates. When we performed a sundar experiment on atomic helium, under the same expenmental condttions, transitions to states wrth n = 40 were observed wrth no indication of sigmficant electnc field nuxmg.
3. Observations A portion 204
of our data, between
3003 and 3008 A
1 September 1980
PHYSICS LETTERS
is shown m fig. 1. The most readily identified resonance lines in fig. 1 are those of a Q branch transition, involvmg the essentially unperturbed levels of a llg state. Because He2 contains two identical zero spin nuclei, Q branch transitions for even rotational levels are non-existent. Any triplet splitting is below the experimental resolution. Thus each group of AN = 0 excrtations terminatmg on a grven electronic excited state forms an easdy assigned Rydberg series. We have been able to tentatively identify members of thrs series up to tz = 25 using a standard Born-Oppenheimer analysis with constants extrapolated from the lower lying electronic states. We fiid that thrs method predicts all observed Q transitions up to IZ = 25 to wrtlun f 0.5 cm-t _ To our surpnse, the u’ = 1 * u” = 1 transitions were nearly as strong as the u’ = 0 f u” = 0 transitions, for n 2 20. (Jndeed, the Q branch lines m fig. 1 are u = 1 + 1.) We have not been able to determine whether thrs is due to our source producing a non-equlbrium drstnbution of vrbrationally excited metastable states, or to possible increases in our detectron efficiency with vrbratronal excitation. Nonetheless, we have observed the transltrons @prr) 3JJg(u = 1) + (2su)a 32i (u”=l)forlO
0
6007
5
33281 7
~A&‘)
60;7
utcm-‘1
33226 4
FIN. 1. IoruzaUon signal versus exaration
5
wavelength. ha&i) IS the wavelength of the undoubled dye laser, “(cm-‘) is the correspondmg transltion frequency. The Q branch m&cated isforthe(11p7r)u3t$ (u’=I)+(2sa)a32~ (u”=l)trans~tion (the stars mdcating nbrationally excited states). Other P and R branch transitions are observed as weU as, of yet, umdenticd hnes
CHEhiICAL PIiYSICS LETTERS
Volume 74. number 2
sitions are at wavelengths which are obscured by the strong atomic helium 2 3S-5 3P transition. In the wavelength range of our investigation there are two possible 523 = 1 off-diagonal wbrational transitions. We were able to observe the (Splr) s 3lTt (u’ = 1) f (2s~) a 3X: (v” = 1) transitions w&rout diffkx&y around h = 2925 A. However, we were unable to observe the (npn) r 3ZZg (u’ = 1) + (2s~) a 3 Xi (v” = 0) transitions around X = 2975 A. The positions of the P and R branch np transrtions are not so easily predicted and assigned as the Q branch. This is due to the strong perturbations between the IZ+ and lI+ states which render a standard spectroscopic analysis completely inadequate_ The P and R branch lines begin to overlap the Q branch series above n = 11, and by n = 16 the Q branch spectrum becomes quote congested by P and R branch lines. In fig. 2 a portion of the recorded spectrum between 2950 and 2955 A is shown where the Q branch spectrum of n = 16 is intermixed with P and R branch lures from levels with 12 in the range 16-2 1. The only tractable method of handling the P and R branch lures is by a MQDT analysis, the assignments shown III fig 2 were derived using a preliminary MQDT analysis based upon the constants of Ginter and Ginter [6 1. A much more thorough MQDT analysis, necessary for the predictron and assignment of the very high n states, IS in progress and wdl be reported at a later date.
i
58534
341230
1 September
L980
i
!ias4 34064B
Fig. 3 Ionization signal versus wavelength. Assrgnments are tentative and serve to indicate the predicted positions for the venous Q branch transitions.
Fig. 3 shows a portron of the observed spectra in the region of n = 25. Many of the transitions observed in fig. 3 are to states that are energetically able to autoionize- For n 2 9 dl excited vibrational(u Z 1) states lie above the u = 0, N = 0 level of the X 2xz state of Her. Many of the u = 0 Rydberg states observed also lie above the u = 0, N = 0 2Ei state of He; due to their rotational energy. Of the states capable of autoio~atio~, some have transitions that appear no broader than inskumentaf, others however appear broadened. Very interestingiy there appears to be Ln some places (see fig. 3) a structured continuum perhaps attributable to Beutler-Fano absorption. When a complete identificationand MQDT prediction of the line positions is compIeted, we should be abk to probe these processesin considerabledetail.
4. Conclusion
59&l
338818
l&ii)
v(crnd)
m*t 338245
Fzg 2. Iomzation signal Y+ISUSextxtahon wavelength over the regon of the Q branch transxtlons (16pd3$ {u’ = 0) +- (2s~)
a3Zt
We have describeda techniqueuseful for the observation of molecular Rydberg states- This technique in two complements optical detection m 7 -t ways. We can observe states with long radiative lifetunes (high n) that cannot bz observedin fluorescence, and we can observe states with datidy shoa kmiation lifetimes due to co&sional or autoionizationprocessesthat againwould not be observedin fluores-
Volume
CHEMICAL
74, number 2
cence. We have observed states m He2 up to II 2: 25 and have identified many new transitrons. Further analysis using an expanded MQDT treatment should allow a full identification of the observed structure as well as a basis for mvestigations of the stabrlity of various levels against autoionization.
Acknowledgement We thank M. Cinter and D. Gmter for conversations and a preprint on optical emission studres of He, Rydberg states and for a careful reading of the manuscript, and V.E. Bondybey for helpful suggestions.
References [ 1]
h1.L Zrmmerman. hl G. Lrttman, bl Kash and D Kleppner, Ph) s. Rev- AZ0 (1980) 215 1, and K P. Huber and G. Herzberg,
molecules (Van Nostrand,
‘06
LEl-l-ERS
refer-
Constants of dlatomlc Prmceton. 1979).
1
September 1980
[3] P.M Debmer and W.A. Chupka, J. Chem Phys. 65 (1976) 2243.66 (1977) 1972. G Herzberg and Ch. Jungen, J. hfol. Spectry. 41 (1972) 42.5. [4] N.W Carlson, private commumcatlon [S] hI.L Gmter, J. Chem. Phys. 42 (1965) 561. M L Gutter and D S Cmter, J. Chem Phys. 48 (1968) 2284; M.L. Gmter and R. Batteno. J Chem. Phys 52 (1970) 4469; hf L. Gmter, J. Chem. Phys 45 (1966) 248, J. Mol. Spectry. 17 (1965) 224, J. Chem. Phys. 18 (1965) 321, F B. Orth and M L Gmter, J. Chem Phys 61 (1976) 282,64 (1977) 223, F B. Orth, CM. Brown and M.L. Gmter. J. Chem. Phys 69 (1978) 53. [6] D S. Gmter and hI L. Gmter, J. Mol. Spectry.. to be pubhhed [7] A B. Callear and R E M. Hedges, Trans Faraday Sot 66 (1970) 2921. [S] W Lchren. hl_V. M cCusher and T-IVienna, J. Chem. F’hys.
ences therem. [2]
PHYSICS
61 (1974) 2200,
T. mema, J. Chem Phls 62 (1975) 2925. [9] T-k howler and V_E Bondybey, J. hfoL Spectry. 78 (1979) 120. [IO] R Panock, R.R. Freeman, J C. White and R-H. Storz, Opt Letters 5 (1980) 160.