SEPTEM HER 1, 1939 PH YSICAL REVIEW

Elementary Processes in the Sensitizetl Fluorescence of OH Molecules

ELISABETH REED LVMAN

Department of Physics, University of Illinois, Urbana, Illinois

(Received July 10, 1939)

The processes involved in the sensitized fluorescenceexcitation of the OH molecules have been investigated byphotometric measurements of the intensities. of lines of the(00), (1,0) and (1,1) OH bands. Collisions betweenmetastable 'P0 mercury atoms and water molecules produceunexcited OH molecules which are then excited by furthercollisions with 3P0 mercury atoms to levels whose energiesare equal to or less than the energy of the 3PO mercury

atoms. Abnormal rotational energy is produced in theexcitation of the OH molecules. Collisions between nitro-gen and OH molecules transfer vibrational energy of theOH molecules to rotational energy and also reduce therotational energy of the OH molecules somewhat towardthermal values. Collisions between helium and OH mole-cules are very much less effective in transferring energythan nitrogen collisions.

INTRQDUcTIoN

HE effect of elementary processes such ascollisions on the rotation of molecules gives

information about the transfer of energy betweenmolecules. Some molecules exhibit abnormalrotational energy characteristic of a temperatureof several thousand degrees even when themolecules themselves are expected to have kineticenergy corresponding to room temperature.These molecules may lose energy through col-lisions. Gaviola and Wood' hrst observed anabnormal rotational energy in the Huoresceneespectrum of HgH. Rieke' has made a study of themolecular processes involved in the production ofthe excess rotational energy of the HgH moleculeand the effects of collisions with foreign gases onthe rotation. An example of abnormal rotationreduced by collisions, although less conspicuousthan the HgH ease, is in the band spectrum ofhydrogen excited by electron impact. Addition ofa small amount of helium to the hydrogen asdiscovered by Richardson' and further investi-gated by Roy4 gives abnormal rotational energyto the hydrogen molecules which disappearswith an increase in the helium pressure.

The spectrum of the OH molecule showsabnormal rotational energy when it is excited in

discharge tubes. ' Bonhoeft'er and Pearson' stated

~ Experimental work carried out at the Department ofPhysics, University of California, Berkeley, California.' E.Gaviola and R. W. Wood, Phil. Mag. 6, 1191 (1928).

~ F. F. Rieke, J. Chem. Phys. 4, 513 (1936).30. W. Richardson, Proc. Roy. Soc. London 111, 720

(1926).4 A. S. Roy, Proc. Nat. Acad. Sci. 19, 441 (1933).~ E. R. Lyrnan, Phys. Rev. 53, 379 (1938).' K. F. BonhoeKer and T. G. Pearson, Zeits. f. physik.

Chemic 14, 1 (1931).

that the source of such energy arises from theprocess of simultaneous dissociation of H20 andexcitation of OH by electron impact. Oldenberg'showed experimentally by a comparison of theabsorption spectrum of normal rotation with theabnormal emission spectrum that the abnormallyrotating rnolecules do not originate from the OHmolecules normally present in the discharge, butfrom the sir'nultaneous dissociation and excitationprocess. After radiation the molecules lose theirexcess rotational energy through collisions.

Since the process of excitation in electricdischarges is complicated, a further key to theexplanation of the abnormal rotation of the OHmolecules might be found in a study of thespectrum excited under more simple conditions.Flame and arc excitation (thermal types ofexcitation) show distributions of rotationalenergy representing thermal equilibrium asexpected, but sensitized fluorescence excitationexhibits the characteristic abnormal rotation ofthe discharge tube spectrum. In fluorescenceexperiments the elementary processes can betraced in great detail. The following investigationof the rotational energy distribution in thesensitized fluorescence spectrum of OH, there-fore, was undertaken in an attempt to explain thesource of the excess rotational energy of the OHmolecule, and to determine some informationabout the exchange of energy between OHmolecules and foreign gas molecules in collisions.

EXPERIMENTAL

A quartz fluorescence tube was used similar in

shape and dimensions to that used by Rieke' in~ O. Oldenberg, Phys. Rev. 46, 210 (1934).

SENSITIZED FLUORESCENCE

his work with HgH. The tube was flattened inthe central portion to reduce the absorbing layerof mercury vapor in the fluorescent region. Twovertical quartz mercury arcs were placed one oneither side of the flattened portion of the fluores-cence tube which was held horizontally. Themercury arcs were watercooled, were each in amagnetic 6eld, and were run at low currentsbetween 1 and 2 amperes. These precautionsguarded against self-absorption of the resonanceline X2537. An image of the cross section of thefluorescent region was focused on the slit of thespectrograph by a quartz lens.

The source of mercury vapor was a drop ofmercury placed in the tail end of the fluorescencetube. In some experiments water vapor wasadmitted through a double stopcock arrange-ment to obtain the desired pressure, and in

others continually flowed through the tube. Inthe nonflowing vapor experiments sufFicient

hydrogen to quench the fluorescence did notseem to be produced by the dissociation of thewater during the time necessary for exposures.

Three groups of photographs were obtainedwith mixtures of mercury, water vapor andnitrogen; mercury and water vapor; and mercury„water vapor and helium. The spectrographused was the 21-foot 30,000-line/inch aluminum

grating in the Paschen mounting. Photographs ofthe (0,0), (1,0), and (1,1) bands were taken in thefirst order. Exposures varying from 1 to 2 hourswere necessary with Eastman 40 plates. Theplates were developed in Rodinal. The method ofcalibrating and measuring the intensities of thelines on the plate was described in a previouspublication. ' The lines of the (0,0), and (1,1)bands were identi6ed from data of Heurlinger, 'and of the (1,0) band from data of Watson. '

RESUI TS

The relative intensities of the measured lines

of the Q~ branch of the (0,0) band are plottedagainst the corresponding rotational quantumnumbers, J', (Fig. 1).The gas in the fluorescencetube was 3.5 mm of water vapor and 10 cm ofnitrogen. The shape of the distribution curve,typical of all OH fluorescence distribution curves

'T. Heurlinger, Dissertation (Lund, 1918).' W. W. Watson, Astrophys. J. M, 145 (1924).

I l l I'

I

D

IIs S- TT Sy ll~ IS- ls- I2- IS—I I I I I I I

e a S S S 2 a

FIG. 1. Intensities of lines of the QI branch of the X3064band from the sensitized Huorescence spectrum as a func-tion of the rotational quantum number. 3.5 mm watervapor a.nd 10 cm nitrogen in the Auorescence tube,

"L.T. Farls, Phys. Rev. 48, 423 (1935).

obtained, is not thermal since there is an excessof rotational energy among the upper rotationalstates and a sudden drop-off of intensitiesstarting at J'= 17-', . The Q& branch is used as anexample of ail the branches in the (0,0) bandbecause it is the branch least perturbed byoverlapping lines. Graphs for the other principalbranches in the band showed the same excessrotational energy in the upper rotational levelswith sharp decreases in intensities beginning withlines arising from the level J'= 17-', . In the (1,0)band a similar drop-off of intensities was foundfor lines originating above the rotational levelJ'=11-,'. Regardless of the mixtures of gases in

the fluorescence tube, the spectrum alwaysshowed the sudden decrease of intensity towardzero above the levels J'=17~ for the (0,0) bandand J'= 11-', for the (1,0) band.

Assuming thermal equilibrium among themolecules, there exists a linear relationshipaccording to the Maxwell-Boltzmann theorybetween the logarithms of Iji and the corre-sponding rotational. term values, T(X'), where Iis the measured intensity of each line, and i thetheoretical intensity factor. The theoretical in-

tensity factors were calculated from Earls'"

468 ELISAHETH REED LYMAN

la'

lo I

IOOOl

5000

T(K)os '

FIG. 2. Rotational term values as a function of log I/i forwater vapor-nitrogen mixtures.

adaptations of the Hill and Van Vleck" formulasfor the 'Z, 'll transitions and the term values forthe (0,0) band were calculated from data ofHeurlinger. Graphs of log I/i vs. T(X') for the(0,0) band illustrate clearly the sudden decreaseof intensities above J'=17-,'. Fig. 2 shows such

graphs for three different conditions in thefluorescence tube. The curved line through thepoints indicates that no thermal equilibriumexists among the rnolecules. In the case of a small

amount of water vapor alone, there is a largeamount of excess rotational energy among thehigh levels. Addition of a small amount ofnitrogen reduces the excess rotational energysomewhat, and addition of a greater pressure of

nitrogen reduces the excess even more towardsthermal values. A straight line drawn throughthe first few points in the latter case has a slopefrom which a temperature of 330'K may becalculated. This is of the order of room tempera-ture. Addition of small amounts of helium in thefluorescence tube showed no effect on the reduc-

tion of rotational energy comparable to the effectof the addition of nitrogen.

There is a slight enhancement of the lines justbefore the sharp decrease in intensities. Thelines arising from around the level J (p p) =12-,' arealso enhanced, especially in the case with thegreater amount of nitrogen in the fluorescencetube. Both cases of enhancement of intensitiesare noticeable in other branches besides the Qq

branch.If the intensity of the strongest line of the

(0,0) band is called 100, and the intensities ofcorresponding strong lines in the (1,0) and (1,1)bands are compared to this line, an approximateidea of the relative intensities of the bands inrelation to one another can be obtained, i.e. ameasure of the respective amounts of vibrationalenergy may be obtained. Table I gives thesedata. It is noted that the intensities of the (1,1)and the (1,0) bands are weak compared to the(0,0) band when there is an increase in theamount of nitrogen present. There is a largeamount of vibrational energy in the (1,0) and the(1,1) bands when a small amount of water aloneis in the fluorescence tube. Helium seems to beless effective than nitrogen in reducing vibra-tional energy.

(1,0)X2811

3.5 MM 3.5 MM 3.5 MM 3.5 MM 3.5 MMH20 H20 H20 Hso Hzo

+10 CM +5 CM +1.5 CM +0.5 CM +1 CM 1 MM

N2 Ng N2 N2 He H&0

8 8 19 45

(1,1))3122

13 24 50

INTERPRETATION OF RESULTS

In the process of sensitized fluorescence, the'P& mercury atoms formed by the absorption ofthe resonance radiation X2537 are very effectivelytransferred to the metastable 'Pp state bycollisions with water or nitrogen. ' According toBeutler and Rabinowitch, " the OH moleculesare formed in the following manner:

Hg (6'P,) +H,O~HgH+OH —0.1 v. (1)

TABLE I. Intensities of strongest lines. Strongest line of(o,o) =10o.

"E. Hill and J. H. Van Vleck, Phys. Rev. 32, 250(1928).

"H. Beutler and E. Rabinowitch, Zeits. f. physik.Chemic BS, 231 (1930).

SENSITIZED FLUORESCENCE 469

l 2536

39432 cv '

L/

12655.8

37642 cs

~17&IIII

I

Il2'j

I

II

I

II

12II

SW44c~-'

If

III

IIIII,I1

l

I

II

375Pt c~-I

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Fio. 3. Vibrational and rotational levels of the OH molecule, and the lower energy levels of the Hg atom.

Since there is a deficiency of energy in (1) andyet OH molecules are known to be formed, theexcess energy must be made up from thermalenergy of the reacting molecules. The OH mole-

cules, therefore, must be formed in the groundstate, 'II3/2, $/2 with little if any excess rotationat this point. Since experimental evidence showsthat excited OH molecules in sensitized fluores-cence do exhibit abnormal rotational energy, theexcess rotation must be acquired, as in the case ofHgH', in the excitation process of the OHmolecule:

Hg(6'Po)+OH~Hg(6'So)+OH' (2)

or in further collisions during the lifetime of theexcited molecule.

The sudden decrease in intensities previouslydescribed indicates that few tran'sitions fromlevels above J'=17-', for the (0,0) band andJ'= 11-', for the (1,0) band are possible, i.e. fewmolecules can be excited above J (p p) =17~ or

J (],p) =11-,'. The energy corresponding to thelevel J (p p)=17~ is 37,444cm 'andto J'(y, p)=11~,37,501 cm '. The energy of the 'Pp mercuryatoms is 37,642 cm '. To the latter value must beadded the thermal energy, kT, which is 210 cm 'at 300'K, i.e. , Hg('Po) lies at 37,852 cm '. Fig. 3is an energy level diagram for the OH moleculeand the mercury atom. The levels J (p, p) =17-',and J'(~, p)

——11-', lie below the 'Pp mercury leveland J'(p, p)

——18-', and J'(~ p)——12-', lie above it. The

maximum amount of energy that can be given tothe OH molecule when it is excited by collisionswith 'Pp mercury atoms is, therefore, the energyof the 'Pp mercury atoms. The fact that a fewtransitions (see Fig. 2) from levels aboveJ'(p, p) =17~ do occur can be explained by thepresence of a few molecules raised to those statesby thermal energy alone. The apparent violationof the law of conservation of angular momentumevidenced in the enhancement of the levelsJ'(p, p) = 16&, 17-,' may be explained as due to thelarge cross section for energy transfer between

470 EL I SAB ETH REED LYMAN

molecules in states with nearly the same energyas the 'Pp mercury level.

When nitrogen is present, collisions of thesecond kind can take place between nitrogenand the excited OH molecules. The energy inexcess of electronic energy is redistributed overthe degrees of freedom of the OH molecule, andvibrational energy is, therefore, transferred torotational energy. The reduction of intensity ofthe (1,0) and (1,1) bands with respect to theintensity of the (0,0) band with increasing pres-sures of nitrogen (Table I) indicates a reductionof vibrational energy. In Fig. 2 a reduction ofrotational energy toward thermal values existswith increasing pressures of nitrogen present inthe fluorescence tube. Collisions with nitrogencan remove molecules from higher to lower v' andJ' states. The nitrogen molecules can take awayrotational energy from the OH molecules. Theexcess rotational energy in the upper states is notreduced a very great amount, however, bycollisions with the nitrogen. There are probablytwo processes occurring simultaneously duringthese collisions, a transfer of molecules from highto low rotational states of the v'=0 level and atthe same time a transfer of molecules from highervibrational states to high rotational states of thezero vibrational level so that excess rotationalenergy still exists among the upper states.

A possible explanation of the enhancement oflines arising from the level J (p p) =12~~ or fromnearby levels of the zero vibrational state arisesfrom the hypothesis of the transfer of moleculesin collisions with nitrogen from the highervibrational states to the high rotational levels ofthe zero vibrational state. The level J (p, p) =12-,'lies approximately at the same place in theenergy level diagram (Fig. 3) as v' = 1.From theseexperiments there is evidence of a loss of excessrotation by collisions during the lifetime of theexcited state. Such transfers of energy occur mostefficiently for small energy changes. One wouldexpect, therefore, an accumulation of moleculesin the low rotational states of the first vibrational

level, v'= 1. Hence transfers of the energy of onevibrational quantum into rotation should bemore probable than transfers of nearby amounts.The result of these transfers is the enhancementof the lines originating from the v'=0, J'=12~level. The more collisions possible during thelifetime of the excited molecules, the greater isthe enhancement to be expected. Such an effectis observed as the pressure of nitrogen in thefluorescence tube is increased (Fig. 2).

CoNcLUsIQNs

The correspondence of the energy of themetastable 'Ep mercury atoms and the energies ofthe rotational states above which the OH mole-cules are not excited in sensitized fluorescenceindicates that the OH molecules obtain theirexcitation energy from, collisions with the 'Ppmercury atoms.

The relatively small effect of collisions withforeign gases, even with nitrogen, on the distri-bution of energy among the excited OH moleeulesshows that the abnormal rotational energy of theexcited OH molecules is acquired in the excitationprocess of the OH molecule. There is definiteevidence, however, that there is a transfer ofvibrational energy into rotational energy as wellas some reduction of rotational energy as a resultof collisions between nitrogen and excited OHmolecules. Collisions between helium and OHmolecules are very much less effective thannitrogen collisions.

The present experiments have no bearing onthe results of the work on the discharge tubeexcitation of the OH molecules. The origin of theabnormal rotation in the OH spectrum excited inelectric discharges is a different process from thatin sensitized fluorescence. ' '

In conclusion the author wishes to express hergratitude to Professor F. A. Jenkins for hisinterest and suggestions during the experimentalpy, rt of this research, and to Professor G. M.Almy for many helpful discussions concerning theinterpretation of the results.