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Supersonic expansion cooling of electronically excited OH radicals
SUPERSONK
8 April 1963
CHEMICAL PHYSKS LETTERS
\‘olume 96. number 3
EXPANSION
COOLING OF ELECTRONICALLY
EXCITED OH RADICALS
A.-l-. DKOEGE and P.C. EirfGELKING
Supcr.ur~x nuzzle woiing c~fcxcitcd-srrlre molsculw radicals produced in a corona discharge is demonstuted and in\ck:i;.~tcd using hydro\j I rJdlc&. Ror~rional rcmperatures of I1 _ + 2 I; and tibrational temperatures of 3400 + 300 K are 1ypk-.dI? obux\rd in rhc 011 A ‘Z+ SIJIC.
cals by an electric discharge in the nozzle throat. in casts dispensing with the need for a high-powered laser. Further. the mecfmisn of cooling appears to operate differently on the various internal degrees of freedom, and we show this here. It is generally difficult to strike a uniform discharge in a high-pressure gas, and when struck, usually an arc form in place of the diffuse glow that is characteristic at low pressures [5]. CJnly on the order of 0.1 W of power is necessary to dissociate molecules in a 100 PI diameter nozzle: thus an inexpensive. low-power highvoltage supply could supplant the laser if an efficient method of maintaining the discharge were available. A corona discharge is such a method.
I_ Inrroduction
many
.-\ number c,F techniques r.idicak. including
~~~s-pk~se -_
IlldJSlJhkS.
tron ~nipact
IX+? he used to prodllcc pyrolysis. reactions with 9IOIll abstractio11. photolysis. and elec[ 1 1. Uni~xruri~tely. n10s.t of these tech-
nques prodwc nmtionally hot products. often wdcsir.rblc for spccrroscopic or reaction studies. We repnrf J siiilple nlcIhod for producing in 2 1nolecu13r IC’Il~>tation.Jl> cold ground and excited SI~ICS of IIIOkcul.ir radicals. tk~th ground and rxitcd stales of rotationally coici pusilivc wns kne b een observed in posl-e.~pansion
clcLtron irnpxt on molecu1.m beams [2]_ Cold neutral rJdlc.J> are not readily produced by this technique beCJIISC tllc drpJrrurr ofhca~~ atomic species from the prccu~sor. as opposed to a light electron in the case of iw:t/Jtlotl. imparts a signillcant recall that must be
2. Experimental
.d~sorbed b\ ths rJdica1 thus produced.
Fig. 1 shows schematically how this is done. A glass is made by a technique similar to that described by Kay et al. [6]_ A 6 nun thick-walled glass tube is
This recoil is and lranslational motion 01‘ the radical, and prevents use of post-expsnsion r.tdicai gencrarion JS a general Inems of producing gold rdiktls. (‘old rJdicJ1 species h.~e becri produced in a supersonic SOIIICC by photolyzing the radical precursor in the r~ouic throat u ith a laser [3_4)_ If the radicals are sufficiently dilure in a11 inert carrier gas, and relasation is f2st comparrd to radical destroying reactions, this ap pedrs 10 be ~11 attrxtive mech&sm for producing lowteInperJture niokcul~r radicals. Nevertheless. we have ~LWII~ and rested a simple method for producing radip~11iI~weciin
316
rotatloudl.
kibration&
results
nozzle
flame heared until closure is achieved. Subsequently, the tube is ground back until a suitable diameter openkg is formed, typically 2% 100 pm. This is fastened in the vacuum apparatus with an Ultra-Torr * fitting. A small gauge tungsten wire is introduced through a “tee” fitting attached to the glass tube. and positioned with one end just behind the nozzle aperture on the high-pressure side. When this wire is energized with
* CJjOn Co.. 32550 Old South Miles Road, Solon. Ohio.
0 009-26 14/83/0000-0000/S
03.00 0 1983 North-Holland
CHEMICAL PHYSICS LJTl-TERS
Volume 96, mrmber 3
100
8 April 1963
01,(2) in each band. These originate fromJ’ = l/2 lev& of v’ = 0 and v’ = 1, the lowest levels in each rotational stack. The almost complete absence of other yc
lines indicates rotational cooling of the radical. The intensities of the P1( 1) and P, (2) lines in both the O-O and l-l bands indicate a rotational temperature consistent with 11 2 2 K. The relative u’ = 1 intensities are consistent with a vibrational temperature of 3400 + 300 K.
Fii.
1. Schematic
of corona
discharge
exited
nozzle.
high voltage (typically 15 kV through stabilizing resistors limiting current to 50 PA), a small corona discharge forms at the tip of this tungsten wire. The current passes through the gas in the nozzle throat. making connection to ground by ionic and electronic conduction in the vacuum on the opposite side of the nozzle. Typical of radical spectra we have observed, fig_ 2 shows a spectrum of the O-O and l-l bands of the A ‘F-X zll emission from electronically excited OH produced by dissociating =I 8 Torr Hz0 in 4 atm of He passing through a 35 .um nozzle- The emission from the region x0.5 mm in front of the nozzle is imaged on the 100 pm slits of a 1.26 m Spex monochromator and detected with an RCA CA3 1000 photomultiplier_. Notice the three strong lines’ PI(l), PI,(l) and * Noration
is consistent
with that of Dieke and Crosswhite
171.
O-O bend
p,(l)
1 p,(2)
q-
_
308
3i4
310
rvoveleng:h
316
Cnrr)
Fig 2. The O-O and l- 1 bands in the A-X emission of exOH radicals, taken under pandon cooled A 22+ excited-state equivalent conditions. Peaks labeled P,(l), P,,(l), 2nd 0,201 correspond lo transitions from the lowest rotational levels in LJ*= 0 and u’ = 1 in the respective band% Rotational temperature is 11 t Z I(. while vibrational temperature is 3100 f 300 K, as obrained from the relative intensities in the spectra.
3. Discussion Two alternative methods of producing rotationally cold excited-state OH appear possible_ As one possibility, the OH radicals produced in the nozzle throat are first cooled by expansion, then excited by a low-momentum transfer mechanism, such as electron impact.
to electronically excited, rotationally cold levels from which they radiate. Since electron impact, or other excitation mechanisms. would impart one or more units of angular momentum (consistent with dipole or
higher mu!tipole) in a significant fraction of the excitations, the noticable deficiency of population in rotational levels above J = l/2 is evidence
that this post-
expansion excitation occurs rarely. As another likeiihood, the discharge produces hot OH in both ground and excited states. which subsequently collisionally relax in the expansion_ Since the radiative lifetime (0.69 11s) [S] is greater than the 0.5 JIS flight time, excited radicals made in the nozzle are likely to be in the post-expansion region we observe. To be consistent with the observed spectra, rotational relaxation must be fast compared to the 0.69 ps radiative lifetime, while vibrational relaxation must be comparably slow. We believe that this mechanism accounts for the majority of the cold OH radicals. Interestingly, while the higb vibrational temperature of the radicals is comparable to the temperature (3500 K) one would expect in the nozzle throat if the dissociating power were equipartitioned over the mass flow. the observed rotational temperature is smaller than that obtainable by expansion cooling from such a hi:& “average” temperature [9]. Because a relatively large portion of the discharge power is deposited in molecular components, owing to their higher electron impact cross sections for momentum transfer and excitation [lo], thermal equilibrium between translational. rota317
Volume 96. number 3
CHEMICAL PHYSICS LETTERS
tional. and vibrational modes would be unlikely in the discharge, and the originally higher vibrational temperatures expected in the discharge would enhance the asymmetry of the rot3tional and vibrational temperatures 3s observed. Thus, even in the nozzle throat. an “werage temper3ture” is an artificial concept. and both discharge 3nd expansion are conducive to low rotational temperatures. These results 3re typical of the cooling of both gou11d .md exited states of other molecular radic3ls obt.rinahle in the coron3 disch.trge. We are currently using this cold r,tdical production technique in contunctwn with laser-induced fluorescence to study polyatomic r,tdic.tl ground and excited state.
Acknowledgement We xkn~~rledge support from the National Science Fcnmdatton. the Department of Energy. and the ,\lurdocl. Foundation during this work.
8 April 19S3
References [ I] D.W. Setser. ed., Reactive intermediates in the gas phase (Academic Press. New York. 1979). [ 2] T-A. Miller. B.R. Zegarski, T.J. Sears and V.E. Bondyhey. J. Chem. Phys. 84 (1980) 3154. [ 3 1 D.L. Monts. T.G. Dietz. M.A. Duncan and R-E. Smalley, Chem. Phys. 4.5 (1980) 133: D.E. Powers. J.B. Hopkins and R.E. SmalIey. J. Phyr Chem. 85 (1981) 2711. [4] A.T. Droege and P.C. EngeIkin_e. unpubEshed. [ 5 1 A. van Lngel. Ionized gasses (Oxford Univ. Press, London, 1965). 161 B.D. Kay. T.D. Lindenman and A.W. Castleman Jr.. Rev. Sci. Instr. 53 (1982) 473_ [ 71 G.H. Dieke and H.\I. Crosswhite. J. Quant. Spectry. Radiat. Transfer 2 (1962) 97. [ SJ K-R. German. J. Chem. Phys. 62 (1975) 2584; 63 (1975) 2512. [ 91 I1.N’. Licpmann and A. Rosko. Elements of _easdynamics t\Viley_ ;r;e\\ York. 1957) p. 10: It. Ashkenkas and F.S. Sherman. in: Rarefied g3s dynxnits, 4th Symposium, ed. J.H. Leeuued (.Acddemic Press. New York. 1966). [ IO] J.L. P.rck. K.E. VoshaII and A.V. Phelps. Phys. Rev. 127 (1962) 2084: Y. ttakalra. J. Phys. Sot. Japm 36 (1974) 1127: G. Sen_e and I_ Linder. J. Phyr B9 (1076) 1.539: K;. Rohr. J. Phys. BlO (1977) 1735; D. Andrick and A. Bitsch, J. Phys. B8 (1975) 393.
8 April 1963
CHEMICAL PHYSKS LETTERS
\‘olume 96. number 3
EXPANSION
COOLING OF ELECTRONICALLY
EXCITED OH RADICALS
A.-l-. DKOEGE and P.C. EirfGELKING
Supcr.ur~x nuzzle woiing c~fcxcitcd-srrlre molsculw radicals produced in a corona discharge is demonstuted and in\ck:i;.~tcd using hydro\j I rJdlc&. Ror~rional rcmperatures of I1 _ + 2 I; and tibrational temperatures of 3400 + 300 K are 1ypk-.dI? obux\rd in rhc 011 A ‘Z+ SIJIC.
cals by an electric discharge in the nozzle throat. in casts dispensing with the need for a high-powered laser. Further. the mecfmisn of cooling appears to operate differently on the various internal degrees of freedom, and we show this here. It is generally difficult to strike a uniform discharge in a high-pressure gas, and when struck, usually an arc form in place of the diffuse glow that is characteristic at low pressures [5]. CJnly on the order of 0.1 W of power is necessary to dissociate molecules in a 100 PI diameter nozzle: thus an inexpensive. low-power highvoltage supply could supplant the laser if an efficient method of maintaining the discharge were available. A corona discharge is such a method.
I_ Inrroduction
many
.-\ number c,F techniques r.idicak. including
~~~s-pk~se -_
IlldJSlJhkS.
tron ~nipact
IX+? he used to prodllcc pyrolysis. reactions with 9IOIll abstractio11. photolysis. and elec[ 1 1. Uni~xruri~tely. n10s.t of these tech-
nques prodwc nmtionally hot products. often wdcsir.rblc for spccrroscopic or reaction studies. We repnrf J siiilple nlcIhod for producing in 2 1nolecu13r IC’Il~>tation.Jl> cold ground and excited SI~ICS of IIIOkcul.ir radicals. tk~th ground and rxitcd stales of rotationally coici pusilivc wns kne b een observed in posl-e.~pansion
clcLtron irnpxt on molecu1.m beams [2]_ Cold neutral rJdlc.J> are not readily produced by this technique beCJIISC tllc drpJrrurr ofhca~~ atomic species from the prccu~sor. as opposed to a light electron in the case of iw:t/Jtlotl. imparts a signillcant recall that must be
2. Experimental
.d~sorbed b\ ths rJdica1 thus produced.
Fig. 1 shows schematically how this is done. A glass is made by a technique similar to that described by Kay et al. [6]_ A 6 nun thick-walled glass tube is
This recoil is and lranslational motion 01‘ the radical, and prevents use of post-expsnsion r.tdicai gencrarion JS a general Inems of producing gold rdiktls. (‘old rJdicJ1 species h.~e becri produced in a supersonic SOIIICC by photolyzing the radical precursor in the r~ouic throat u ith a laser [3_4)_ If the radicals are sufficiently dilure in a11 inert carrier gas, and relasation is f2st comparrd to radical destroying reactions, this ap pedrs 10 be ~11 attrxtive mech&sm for producing lowteInperJture niokcul~r radicals. Nevertheless. we have ~LWII~ and rested a simple method for producing radip~11iI~weciin
316
rotatloudl.
kibration&
results
nozzle
flame heared until closure is achieved. Subsequently, the tube is ground back until a suitable diameter openkg is formed, typically 2% 100 pm. This is fastened in the vacuum apparatus with an Ultra-Torr * fitting. A small gauge tungsten wire is introduced through a “tee” fitting attached to the glass tube. and positioned with one end just behind the nozzle aperture on the high-pressure side. When this wire is energized with
* CJjOn Co.. 32550 Old South Miles Road, Solon. Ohio.
0 009-26 14/83/0000-0000/S
03.00 0 1983 North-Holland
CHEMICAL PHYSICS LJTl-TERS
Volume 96, mrmber 3
100
8 April 1963
01,(2) in each band. These originate fromJ’ = l/2 lev& of v’ = 0 and v’ = 1, the lowest levels in each rotational stack. The almost complete absence of other yc
lines indicates rotational cooling of the radical. The intensities of the P1( 1) and P, (2) lines in both the O-O and l-l bands indicate a rotational temperature consistent with 11 2 2 K. The relative u’ = 1 intensities are consistent with a vibrational temperature of 3400 + 300 K.
Fii.
1. Schematic
of corona
discharge
exited
nozzle.
high voltage (typically 15 kV through stabilizing resistors limiting current to 50 PA), a small corona discharge forms at the tip of this tungsten wire. The current passes through the gas in the nozzle throat. making connection to ground by ionic and electronic conduction in the vacuum on the opposite side of the nozzle. Typical of radical spectra we have observed, fig_ 2 shows a spectrum of the O-O and l-l bands of the A ‘F-X zll emission from electronically excited OH produced by dissociating =I 8 Torr Hz0 in 4 atm of He passing through a 35 .um nozzle- The emission from the region x0.5 mm in front of the nozzle is imaged on the 100 pm slits of a 1.26 m Spex monochromator and detected with an RCA CA3 1000 photomultiplier_. Notice the three strong lines’ PI(l), PI,(l) and * Noration
is consistent
with that of Dieke and Crosswhite
171.
O-O bend
p,(l)
1 p,(2)
q-
_
308
3i4
310
rvoveleng:h
316
Cnrr)
Fig 2. The O-O and l- 1 bands in the A-X emission of exOH radicals, taken under pandon cooled A 22+ excited-state equivalent conditions. Peaks labeled P,(l), P,,(l), 2nd 0,201 correspond lo transitions from the lowest rotational levels in LJ*= 0 and u’ = 1 in the respective band% Rotational temperature is 11 t Z I(. while vibrational temperature is 3100 f 300 K, as obrained from the relative intensities in the spectra.
3. Discussion Two alternative methods of producing rotationally cold excited-state OH appear possible_ As one possibility, the OH radicals produced in the nozzle throat are first cooled by expansion, then excited by a low-momentum transfer mechanism, such as electron impact.
to electronically excited, rotationally cold levels from which they radiate. Since electron impact, or other excitation mechanisms. would impart one or more units of angular momentum (consistent with dipole or
higher mu!tipole) in a significant fraction of the excitations, the noticable deficiency of population in rotational levels above J = l/2 is evidence
that this post-
expansion excitation occurs rarely. As another likeiihood, the discharge produces hot OH in both ground and excited states. which subsequently collisionally relax in the expansion_ Since the radiative lifetime (0.69 11s) [S] is greater than the 0.5 JIS flight time, excited radicals made in the nozzle are likely to be in the post-expansion region we observe. To be consistent with the observed spectra, rotational relaxation must be fast compared to the 0.69 ps radiative lifetime, while vibrational relaxation must be comparably slow. We believe that this mechanism accounts for the majority of the cold OH radicals. Interestingly, while the higb vibrational temperature of the radicals is comparable to the temperature (3500 K) one would expect in the nozzle throat if the dissociating power were equipartitioned over the mass flow. the observed rotational temperature is smaller than that obtainable by expansion cooling from such a hi:& “average” temperature [9]. Because a relatively large portion of the discharge power is deposited in molecular components, owing to their higher electron impact cross sections for momentum transfer and excitation [lo], thermal equilibrium between translational. rota317
Volume 96. number 3
CHEMICAL PHYSICS LETTERS
tional. and vibrational modes would be unlikely in the discharge, and the originally higher vibrational temperatures expected in the discharge would enhance the asymmetry of the rot3tional and vibrational temperatures 3s observed. Thus, even in the nozzle throat. an “werage temper3ture” is an artificial concept. and both discharge 3nd expansion are conducive to low rotational temperatures. These results 3re typical of the cooling of both gou11d .md exited states of other molecular radic3ls obt.rinahle in the coron3 disch.trge. We are currently using this cold r,tdical production technique in contunctwn with laser-induced fluorescence to study polyatomic r,tdic.tl ground and excited state.
Acknowledgement We xkn~~rledge support from the National Science Fcnmdatton. the Department of Energy. and the ,\lurdocl. Foundation during this work.
8 April 19S3
References [ I] D.W. Setser. ed., Reactive intermediates in the gas phase (Academic Press. New York. 1979). [ 2] T-A. Miller. B.R. Zegarski, T.J. Sears and V.E. Bondyhey. J. Chem. Phys. 84 (1980) 3154. [ 3 1 D.L. Monts. T.G. Dietz. M.A. Duncan and R-E. Smalley, Chem. Phys. 4.5 (1980) 133: D.E. Powers. J.B. Hopkins and R.E. SmalIey. J. Phyr Chem. 85 (1981) 2711. [4] A.T. Droege and P.C. EngeIkin_e. unpubEshed. [ 5 1 A. van Lngel. Ionized gasses (Oxford Univ. Press, London, 1965). 161 B.D. Kay. T.D. Lindenman and A.W. Castleman Jr.. Rev. Sci. Instr. 53 (1982) 473_ [ 71 G.H. Dieke and H.\I. Crosswhite. J. Quant. Spectry. Radiat. Transfer 2 (1962) 97. [ SJ K-R. German. J. Chem. Phys. 62 (1975) 2584; 63 (1975) 2512. [ 91 I1.N’. Licpmann and A. Rosko. Elements of _easdynamics t\Viley_ ;r;e\\ York. 1957) p. 10: It. Ashkenkas and F.S. Sherman. in: Rarefied g3s dynxnits, 4th Symposium, ed. J.H. Leeuued (.Acddemic Press. New York. 1966). [ IO] J.L. P.rck. K.E. VoshaII and A.V. Phelps. Phys. Rev. 127 (1962) 2084: Y. ttakalra. J. Phys. Sot. Japm 36 (1974) 1127: G. Sen_e and I_ Linder. J. Phyr B9 (1076) 1.539: K;. Rohr. J. Phys. BlO (1977) 1735; D. Andrick and A. Bitsch, J. Phys. B8 (1975) 393.