Hanle effect in BO2

V&me

60, number 1

CHXMICAL

PHYSICS ILETTERS

1.5 December 1978

EiANLE EFFECT IN B02 Dzniel COULTER,

Chung-Yung Robert WU and David A- DOWS

Department of Gzemistry. Universi@ of Sn.hem

Gzlifomk

Lxx Angeles, Cdifomia

90007. USA

Rcccivcd 13 July 1978 Revisedmanuscriptreceived5 September1978 Hanieeffect measurementson the BOz A*il, state excited at 514.5 nm are reporte& Dappler-freepressure-and Zeeman-boradening effects are observed.The A21-fUstate is c10s.e to Hund’s case a Collisionaldepolarizationand broadening cross sectionsare detived.

1. Introduction The visible spectrum of BO, has been studied in both absorption [I] and in laser-excited emission [24]. The observed bands in the green arise from the A *IT,-X *Bg electronic transition in the linear symmetric molecule. Hyperfme structure has been resolved by saturation techniques in the (11 ‘O)2Z (01’0)21: vibronic band of the A-X electronic%rnsition in 11BOz [S]. More recently, microwaveoptical double resorrnce transitions have been observed among the Zeeman-tuned rovibronic sublevels of +&e X*“_o state [6] _ We report a study of the Hanle effect in the J= 1512 + 17/2 line of the (11 ‘O)2ZZ~-(O110)22, vibronic band, which is coincident with the 5 145 A tie of the Ar+ laser. We have es&mated the lifetime and the collisional depolarization cross section for the excited *Z state based on a theoretical calculation of the mo Pecularg-factor. We also report the observation of three new rovibronic transitiorr near 5145 A as well as some semiquantitative data on pressure and Zeeman line broadening effects.

2 Experimental In all of the experiments the 5145 a line of a Coherent model 53 G Ar+ laser was used to excite BO, radicals. The laser could be operated mlultirnode, or single mode with an intracavity etalon. The system

could be locked to a single mode for up to several hours by use of a Lansing %80-214 lock-in-stabilizer. Typical output power during the experiments was 1 W multi-mode and 100 mW in single mode operationBOZ radicals were formed in a flow system [2,3] by the reaction of BCl, with oxygen atoms produced by passing molecular oxygen through a microwave discharge. The reactants were mixed in a Pyrex cell through small nozzles approximately 5 mm apart. The total reaction pressure ranged from 0.3-1.5 torr (=% = l-100 p) and was measured near the reaction zone by a Pirani gauge or MKS capacitance monometer. In the Hanle measurements, two sets of concentric Hehnholtz coils were used. Tire primary coils were capable of producing dc fields up to r300 G. The secondary coils provided a small ac field (S-10 G, 400 Hz) to facilitate phase sensitive detection_ The laser beam was coincident with the magnetic field axis. Its polarization direction was controlled by a polarization rotator. The fluorescence was viewed through Pyrex windows at rig& angles to the beam by an Bh4.I#%655R photomultiplier tube. A linear polarizer was used to select only the component of the fluorescence that was poled perpendicular to rhe field axis. Orange glass filters were used to block the scattered light. Magnetic field measurements w&e made with a Bell 120 gaussmeter. The data were digitized and parameters in a theoretical expression for the Hanle effect were determined through standard 51

CHJzWC&L.PHYSICS LJzrl-ERS

Voiurne 60. number 1

least square fitting techniques. I_&e broadening measurements and high resolution spectra were obtained by the saturation spectroscopy technique of intermodulated fluorescence [7], in which two coaxial, oppositeIydirected beams from the same laser are chopped at different frequencies (1080 and 720 I&) and the fluorescence modulation is monitored at the sum frequency. For = 100 -ME& Imsolution, we simply disconnected the power supply to the etdon oven, allowing the laser to jump from mode to mode as the etaion temperature dropped. For narrow line width measurements we obtained a resolution of aI0 MHz by scanning a single cavity mode crossing the laser gain curve in a comimrous manner. This procedure involved tilting the etaton while simultaneously driving the front caviiy mirror with a piezoeIectric translator. Fiuorescence was detected in the same manner as in the Ha.nle effect but with the lockin detector referenced to an 1800 Hz waveform produced by mixing refe_mnce signals from the two choppers- Frequency sczms were caBi_mted agG%t the measured hyperfine structure of I2 [8] _

3_ Results and airalysis The excitaticn spectrum of BO, shows three lines and a weak shoulder in the region of *&e 5145 A gain curve IS] _ These lines have been attributed to the (lllO)%e,J=

17/2-+(01’O)2~,,J=

(lI’O)2A5~2,f

= 51!2 ~(0110)2A~/2,J=49/2

(1220)zo7,2,J=

55j2 -+ (02%)%,,

IS/2 (l’BO2)

I = 5312

roviironic transitions belonging to the A-X electronic transition 131. Ihe inter-modulated fluorescence spectrum at a resolution of ~100 MHz is shown in fig_ 1. Peaks A, B and D correspond to the three previously observed features. When examined at a resolution of -10 MHz peak D shows four clearly resolved components of hyperfime structure as is expected for a low J transition in “BO, On the basis of this observation, Muirhead et al_ 151 have assigned peak D to the X-Z: transition. &aks A and B show no resolved structure at 10 MHz resohrtion which is consistent with an assignment to the high J, A-5 and Cp-Q, transitions [S]. The other three peaks, C, E and F a(e reported here for the first 52

15 Decembar 1978

I

I

I

r

0

2

Q

6

I

-2

I

8 GHt

Fig_ 1. Intermodulated fuorescence. spectrum of BO2 near 5145 -$, at a resoIution of 100 &fHz. Line D is ffisponn%Ie for the Hade effect-

time. Possibly they are derived from higher hot bands and are responsible for weak unassigned features in the fluorescence [2,3] and optical-rPicrowave double resonance spectra 161. The positions of these lines relative to peak A are given in table 1. As has been indicated previously [2], when excited with the 5145 A Iine of an Ar* laser, B02 exhiiits a strong Harrle effect. The normal Me effect signal [9JO] is expected to have a lorentzian shape with a half width at half maximum of

(1) where JQ is the Bohr magneton,g is the molecular I_andeg-factor and r is the excited state lifetime. lbe observed Hanle effect is found almost entirely in the %, excited state. The signal was maximized when a &gle laser mode was tuned ?o the center of the Z-Z line, and the signal observed was identical to that found multimode when normalized to laser output Table I Reti+&=

positions of the six lines observed near 5145 A Peak

Av (kO.1) GHz

A B C D E F

0.0 1.8 3-3 3.9 4.9

5.7

Volume 60. number 1

CHEMICAL

PHYSICS

power. An indication of a magnetic effect was noted when a single laser mode was tuned to the A peak, but it was so weak it could not be characterized. The total observed change in fluorescence intensity during the Hknle experiments was -16%. This is in good agreement with theory [9] for RfRC and Rtl?J transitions. A typical frost derivative line shape, resulting from phase sensitive detection, is shown in fig. 2. ‘lhe incident beam was polarized parallel to the observation direction. We have measured the Hanle line widths as a function of total pressure and these data are shown in fig. 3. A linear least-squares fit of the data yields a zero pressure half width of 32.9 G and a slope of 13.8 GJtorr, both with standard deviations of 1.3 units. Huie et al. [12] have recently reported lifetimes for the A state of B02. Exciting in the 518 run, (100~(000) transition results in fluorescence with a lifetime of 76.3 ns. From this and our zero-pressure H,ir, we obtain an apparent g = 0.0225. However, Kim et ai. [6] suggest that to a good approximation this state is Hund’s case a. The g factor for Hund’s case a coupling is given by g=(A+22)(A+X+i)/J(J+

1).

(2)

where A, Z:, and I are the ccmponents along the internuclear axis of the electronic orbital, electronic spin and vibrational angular momentum. The total angular momentum quantum number is desipated J_ Us.in~ the theoretical Hund’s case ag-factor for the 22, state (0.0124) in eq. (I) we calcuIate a zero pressure Iyfetime of 1.40 X lo-? s. The disagreement between the measured radiative lifetime of the (100) state and the lifetime of the (11’0) state determined from the Hanle effect im-

.

*a L

. . l

1

I

-200

-100 Magnetic

100 0 Field (Gauss)

200

Fig_ 2. First derivative H&e effect obtained at 0.799 torr pressure. The points are calculated from the least-squaresfit to the data.

IS December

LEX-CERS

20!

I

0

3

I 1.0

Total Pressure

Fig. 3_ Hanle line widths H1,2 versus linear fit shown.

1978

I ts

(tom)

pressure, wifi the best

plies that there is either significant variation in lifetime among vibronic states or that the true g-factors are larger than these calculated from eq. (2) There is little to distinguish between these two possibilities. However, in light of the fact that reported lifetimes for several vibronic state agree to within 15% [ 121, it is hard to understand how the (1 I IO) state could live significantly longer. It seems more likely that the g-factors are larger than those expected for case a coupling, which is consistent with Zeeman broadening results discussed below_ It is possible to use the data in fig_ 3 to obtain a cross section for collisional depolarization. Observed lifetimes calculated from Hanle half-widths are related to o by the expression l/T&

= f/-r,,

+ nav,

0)

where rZao is the zero pressure lifetime, rz is the totaI number density of the vapor and v’ls the relative velocity of collision partners. Assuming a velocity of 3.85 X lo4 cm/s we calculate a cross section of 24 A2 ifg= 0.0124 (or 44 A2 ifg= 0.0225). We have measured the linewidths of peaks A and S in fig_ 1 at several pressures with -10 MHz resolution. The line widths (hwhm) are 20-25 M& at zero pressure @robably unresolved hyperfine structure) and increase approximately 5 MHz/torr_ The corresponding cross section for collisional broadening is 40 A*, comparable to that from the Hanle effect. Finally, we report the observation of Zeeman 53

Voiume 60, number 1

CHJZMICALPHYSICS LEITJZRS

broadening [I Z] in peaks A, 13 and D of fig. lUtilizing the intermod&ted fluorescence technique again to measure Iine widths, we found significant (SO-100%) broadening of peaks A and B at fields of -00 G_ The magnitude of the broadening suggests that the trueg-factors in the 4712 and 2A5i2 states may be somewhat iarger than those calculated from eq_ (2)_ Attempts to study +&eeffect of a magnetic field on the hyperfiie components of the “Z Iine have been un&xessfuI. It &as found &&hat alI strutture wzs smeared out at fields as low as 10 G. This is cortistent with the observation of Kim and coworkers [6] who measured g-factors of 0.1-0.3 in the lower electronic state. The significant deviation from case a couplin,o may be a manifestation of Iarge Renner-Teller interaction.

54

15 December 1975

References 111 J-W-C. Johns, can. J- Pbys 39 (1961) 1738_ [2] DX. Russen, Nu.Kroll, D-k Dows and R_AA. Beaudet, chem Plws Letters 20 (1973) 153. [31 DE. RusWI, N. KroB and R-A. Beaudet, J. Chem. Phys. 66 (1977) ‘999. [4] A. Fried and C-W- Mathews, Chem. Phys Letters 52 (1977) 363-

151 A Muirhead, K.V.LN_ Sastry, R.F. Curl, J. Cook and F-K. Tittel, Chem. Phys. Letters 24 (1974) 208. (61 M-S-Kim, R-E_ Smalley and D-H_ Levy, J. MoL Spectry. 71 (1978) 458.

171 M-S. Sorem and A.L. Schawiow, Opt. Commun. 5 (1972) 148.

WI MD_ Levenson and A-L.. Schawlow, Phys Rev. A6 (1972)

IO-

191 RN. Zare, J- Chem. Phys 45 (1966) 451Q [iol R.N. Zare, Accounts Chem. Res. 4 (1971) 361. r111 F-R Crawford, Rev- Mod- Phys. 6 (1934) 90. WI R.E. Huie, NJ-T_ Long and B.A. Thrush, Chem. Phyr Letters 55 (1978) 404.