Lifetime measurements of excited quantum states using a diode laser. Radiative lifetime of the B2∑+ state in BaCl

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LIFETIME MEASUREMENTS OF EXCITED QUANTUM RADIATIVE LIFETIME OF THE B *Z+ STATE IN BaCl G. GUSTAFSSON,

H. MARTIN

15 June 1988

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STATES USING A DIODE LASER.

and P. WEIJNITZ

Institute of Physics, Uwersity ofStockholm. Vanadisv. 9, S-1 1346 Stockholm, Sweden Received

30 October

1987; revised manuscript

received

29 January

1988

A new method has been developed for measuring radiative lifetimes ofexcited molecular and atomic quantum levels. It is based on the unique time characteristics and the high spectral resolution of single mode diode lasers. The diode laser is tuned through the spectral region of interest at a high repetition frequency. With the delayed coincidence method fluorescence decay is recorded from each spectrally resolved line, allowing for lifetime determinations of several sequentially excited levels in each time-resolved spectrum. This method is applied on several rotational and vibrational levels of the B ‘Z+ state of gaseous BaCl. From the measured lifetime, 104 k 3 ns, band oscillator strengths and electronic transition moment were derived for the BaCl B-X transition.

1. Introduction The use of diode lasers as spectroscopic tools is quite common today. In the far infrared part of the electromagnetic spectrum they have been used for more then 15 years. In order to get these far infrared diode lasers to work properly, rather complicated arrangements are needed, since the laser has to be cooled down to low temperatures. Other technical difficulties, for example multimode oscillation, also contribute to the fact that the commercial far infrared diode laser systems are quite expensive. Still, they are attractive tools in absorption spectroscopy compared with alternative measuring systems, such as the even more expensive high resolution Fourier transform spectrophotometers. Quite in contrast to the far infrared devices are the near infrared diode lasers, which are much simpler to engineer. The near infrared lasers are today produced in large quantities primarily for the optics communication industry and for the use in laser printers, compact disc players, etc., and are therefore incredibly inexpensive. These lasers are available as high resolution single frequency lasers, have a low threshold current ( < 50 mA), and work at room temperature without any expensive arrangement. Compared with all other existing types of lasers, they are relatively insensitive to mechanical vibrations and 112

their physical dimensions are extremely small. These features make them very attractive light sources in many different applications. However, the near infrared diode lasers have not yet become widely used in spectroscopy. Perhaps this fact is due to the lack of commercial advertisements addressed to the spectroscopic community, in combination with the rapid development of the modern high power GaAlAs-diode lasers. Some spectroscopic work using near infrared diode lasers has nevertheless been reported in the past. This is reviewed in ref. [ 1 ] for the case of atomic spectroscopy. A detailed description of a high resolution near infrared diode laser spectroscopic system suitable for molecular spectroscopy has recently been reported [ 2 1. With this diode laser system, for instance, three different electronic states in the BaCl molecule were rotationally analysed - the X, B and F 2C+ states - using excitation spectroscopy [ 31 and optical-optical double-resonance spectroscopy [ 41. N: has also been studied with a low-cost near infrared diode laser using the technique of velocity modulation spectroscopy [ 5 1. The diode laser can be modulated by several gigahertz and it is therefore, in many cases, an excellent light source in time-resolved spectroscopy. Only a few such experiments have been reported so far with diode lasers (see for example [ 6,7] ). In this paper,

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discuss a new laser spectroscopic method for measuring lifetimes of excited states, taking advantage of the unique time characteristics of the singlemode GaAlAs laser. This method relies on the extreme reproducibility of a high frequency ac induced laser wavelength tuning. Single photon detection and the delayed coincidence technique is used in order to correlate the deexcitation events with the time base, and simultaneously to correlate the excitations with laser wavelength. A tuning repetition frequency of the order of one megahertz is convenient. The power of this high frequency tuning method in time-resolved molecular spectroscopy is demonstrated in this paper by a study of individual rotational levels of the B 2E+ state in the BaCl radical. For several reasons, gaseous BaCl was chosen as the test molecule for the experiments. The B 2Z+-X ‘C+ transition is well known, is in a convenient wavelength region for low-cost diode lasers, and it has been investigated previously with such devices [2]. Moreover, the radiative lifetime of the B ‘C+ state gas not previously been reported, and is an important parameter when deriving the electronic structure of the low-lying electronic states in BaCl. Much effort has been expended in the past to structurise and understand the different spectroscopic data on the group IIa metal monohalides, with the emphasis on an understanding of the molecular bond for this class of diatomic molecules [ 891. we

2. Experimental 2. I. Laser operation

15 June 1988

der 0.08 cm-‘/mA and can be utilised at high repetition frequencies. If the laser is driven by a high frequency saw-tooth shaped current, a fairly linear high frequency single mode tuning over several wave numbers is possible. The spectral range of the frequency tuning depends primarily on the ac amplitude, but also on the repetition frequency of the driving current. With our experimental conditions we found the dependence shown in fig. 1. The emitting spectral region of the laser could be varied by changing the temperature of the diode or by introducing a dc bias to the injection current. The temperature was thermoelectrically controlled by a Peltier block clamped to the heat sink of the diode laser. For wavelength supervision, part of the collimated laser beam was passed through a 1.5 m monochromator (Jobin Yvon HR 1500) equipped with a 1024 channels photodiode array at the exit slit. In this way the laser frequency could be determined to an accuracy of 0.3 cm- ’ and also the laser single-mode operation could be established in real time. A part of the laser beam also entered a confocal Fabry-Perot interferometer (Spectra Physics 470) with a 2 GHz free spectral range. These instruments admitted a laser frequency stabilisation of about 250 MHz by visual inspection on an oscilloscope and manual feedback of the injection current.

GHz

120 100 -

The laser used in the experiments was a Hitachi HLP 1600 with a laser wavelength centred at 826 nm and a rated cw optical output power of 15 mW. The laser was pulsed by modulating the injection current. Different waveforms were generated by a function generator (Hewlett Packard 8007A). The rise and fall time of the laser is rated to be less than one nanosecond and was in our case limited by the function generator (2.5 ns). Since the injection current directly couples to the thermal expansion of the laser cavity and, hence, to the laser frequency, there is an almost linear relationship between the driving current and the laser frequency. This effect is of the or-

80 -

60 -

LO -

Fig. 1. Diode laser tuning range Av (GHz) as a function of repetition frequencyf( Hz). The laser was a Hitachi HLP 1600. The laser current was a 40 mA ramp modulation on a bias just below the laser threshold current.

113

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2.2. Molecule production and optical arrangement The emitted light from the laser was focussed into a Broida type oven [ 10 ] were the BaCl molecule was formed through a chemical reaction. The oven consists of a resistance furnace connected to a vacuum chamber by a small hole of @= 5 mm. The vacuum chamber was evacuated to a pressure of lop3 Torr by a pump with a capacity of 900 l/min. Metallic barium was evaporated in the resistance element by heating to 800-900’ C. A flow of argon gas was passed through the furnace resulting in a diffusive beam of argon seeded with barium atoms at a total pressure of around 1 Torr. The BaCl molecule was produced by introducing a small amount of HCl from a ringshaped injector. HCl was chosen as the oxidant because it is known to produce BaCl at low vibrational levels and only in the ground state, without any disturbing chemiluminescence [ 8 1. The laser beam was directed towards the molecular flow and focussed at the centre of the vacuum chamber. Perpendicularly the laser-induced luminescence of BaCl was collected by a f= 10 cm lens and focussed on the entrance slit of a 0.2 m monochromator (Jobin Yvon H.20) serving as an optical band-pass filter. This monochromator rejected scattered laser light and is a necessity in optically filtered excitation spectroscopy [ 11,12 1. Since the spectrum of BaCl is very dense different rotational levels from different vibrational modes can be excited in the same laser scan. Therefore, by a partial dispersion of the laser-induced luminescence it is possible to measure the lifetimes of these states separately with the same experimental conditions except for different settings of the monochromator. The monochromator had a bandwidth of several nanometers and was tuned to different B-X bands depending on which vibrational level was currently under study. The laserinduced luminescence was detected with a Hamamatsu R928 photomultiplier tube. 2.3. Lifetime measurements The measurements of the radiative lifetimes were based on the delayed coincidence method. A scheme of the experimental setup is presented in fig. 2. Selected quantum levels were laser excited during a very short time ( - 10 ns at a high repetition rate ( 100 114

Fig. 2. Schematic mental setup.

drawing

of the lifetime

measurement

experi-

kHz) ). This was realised by repetitively tuning the diode laser at a high speed ( - 2 cm- ’ / ps). The average detection rate was kept below 10 kHz in order to assure that not more than one photon was detected in each laser wavelength scan. In a time-toamplitude converter (TAC) (Ortec 437A) the time interval between the laser start trigger pulse and the photomultiplier stop pulse was converted to a pulse height proportional to the delay and was stored in a multichannel analyser (MCA) (Nuclear Data 62) where the spectrum was displayed. The low count rate resulted in undistorted spectra free from pile-up errors. The time-resolution was set to 2.7 ns. A large number of events were collected before the data stored in the MCA were transferred to a computer where the lifetime of the quantum levels were later determined. Since it was possible to tune continuously over several wavenumbers one could determine the radiative lifetime from several spectral lines in each recording. This was possible due to the high reproducibility of the laser scan which assured a constant time delay between the laser start trigger and the different sequential excitations in the tuning. The reproducibility and stability of the laser is so high that the accumulating time can last for tenths of minutes in order to improve the signal to noise ratio. Fig. 3 shows such an example where a 1 cm-’ scan at 100 kHz repetition rate was performed in the spectral region of the B-X ( 1-O) band sequence of BaCl. Also shown in the figure are the laser current as a function of time and, for comparison, the ordinary absorption spectrum of BaCl in the same spectral region. Considering time spectrum overlap, it is of course better to pick out one single quantum level at a time

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COMMUNICATIONS R,(78)

I

I

12114.0

12113.0

cm

-1

Fig. 3. Part of the absorption spectrum of the B ‘I+-X ‘Z+ transition of BaCl (upper trace) together with the high speed tuning laser induced fluorescence (middle trace). The laser diode was tuned 1.2 cm-’ at a repetition rate of 100 kHz in the spectral region of the (1-O) band sequence. The dominating rotational series seen on these two spectra belong to the R,-branch with N= 58-6 1. Also shown is the ramp injection current pulse (lower trace). The pulse length was around 0.7 us.

for lifetime measurements rather than to measure in the crowd of lines. With minute adjustments of the injection current this can be done. The laser wavelength scan is stopped by switching the laser off exactly at one selected spectral line and the last decay is monitored as shown in fig. 3. If one still is concerned about the decaying fluorescence from the previously excited levels it is also possible to drive the diode with, for instance, a square-shaped current pulse. In that case a nonlinear wavelength scanning will be obtained. The laser then tunes very rapidly in the beginning of the pulse, which can be seen from the interferometer fringes in fig. 4. As thermal equilibrium is approached in the laser diode-junction, the tuning will slow down and almost stop. Consequently, it is easy to control the laser so as to excite one specific rotational level at the end of the wavelength scan at a time when all previous excitations have well decayed. In addition, the excitation time can be adjusted for maximum efficiency. Still, with

-

I

Fig. 4. A 2 cm-’ long scan with a square pulse driven diode laser at a repetition frequency around 100 kHz in the spectra1 region of the B-X ( 1-O) band sequence of BaCl (826 nm). The laser tuning is monitored by a 2 GHz free spectra1 range interferometer equipped with a fast photodiode connected to an oscilloscope (inserted below the MCA display of the BaCl time-resolved excitation spectrum).

this technique it is possible to observe several rotational lines in the same scan. Such a spectrum, although on a nonlinear wavelength scale, is very useful since it serves as a record of which spectral line one has actually selected (fig. 4).

3. Results and discussion With the methods described in the previous section, the lifetimes were determined for selected rotational-vibrational levels of the B 2Z+ state in ‘38Ba35C1. Different rotational quantum numbers N in the vibrational levels V= 1 and ~2 were studied for both “e” and “f” levels. Typical examples of timeresolved diode laser excitation spectra of BaCl is shown in figs. 3 and 4. The laser was tuned at different spectral regions of the B-X Au= + 1 band se115

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quence (824-828 nm) while the monochromator was set at the wavelength for the B-X ( 1,l) or the (2,1) transition, that is at 844 nm or 826 nm. From known spectral data of BaCl [ 31 the rotational lines in the recorded spectra were easily identified. In order to determine the radiative lifetimes free from collisional effects several lines were measured at different total pressures in the range 0.1 to 1.8 Torr. The electronic quenching due to the Ar buffer gas was found to be small, around 1 MHz/Torr. When the experiments on one selected level were finished, the next rotational line in the particular spectral region was tuned in and measured. The data were transferred to a computer where nuclear physics software [ 13 ] was used to calculate the exponential decay. The first 10 ns after the laser turnoff were omitted in the calculation, which resulted in statistically limited fittings. No trends could be found in the results as a function of N or v for the derived lifetimes in the measured region 36
15 June 1988

nohalides, i.e. 38 ns for CaCl and SrCl [ 81. The radiative lifetime 7 of the upper state vibrational level v’ is related to the transition probabilities A,..,.. by l/7,,,

= c A,.,. ‘,‘,

)

(1)

where the A,,,,.,, are given approximately A”,,.,.= I&

q,,,,,,R:(r,....)

64n4/3h .

by (2)

In eq. (2) v~,,~.. , ql;,L,,,and r,,,.,, are, respectively, the transition frequency, the Franck-Condon factor and r-centroid of the (v’, v” ) band, and R:(r) is the square of the electronic transition moment. Using the radiative lifetime derived here and the known molecular constants for the X and B state [ 3 ] and the program TRAPRB [ 15 1, the q*,,,,,‘,ru.“..and band oscillator strengths J$~,, were calculated for the B-X bands (table 2). This information is consistent with a squared electronic transition moment R,2 = 3.0 * 0.1 (au) for the B-X transition at an internuclear distance of about 2.7 A. This can be compared with the value Rz = 4.05 IL0.17 known for the BaCl C-X transition [ 8 1. This latter value was established in a systematic investigation of the excited state lifetimes for the monohalides [8] where 17.5 ns and 16.6 ns were found for the BaCl C ‘II state Q= l/2 and 3/2 spin-orbit components, respectively, using the fluorescence decay method. For the lighter alkaline earth monohalides the situation is reversed, that is the squared transition moments are larger for the B-X than for the C-X transitions. This fact can be interpreted as there is an increase of the d-character in the B state valence orbital as the metal gets heavier. With a one electron model and an ionic picture of the chemical bond [ 8,9] analysis of the ground state microwave spectrum shows that the valence electron is predominantly described by a Ba+ 6s wavefunction, with some mixing of 6p and 5d wavefunctions [ 16 1. The Ba+ atomic transition moment is larger for 6s-6p than for 6p-5d [ 17 1. Therefore, the larger transition probability for the C-X transition compared to the B-X transition is consistent with an even larger d character and less p character for the valence electron in the B state, and the opposite for the C state. This is expected since the B 2C, A ‘II and A’ 2A states are supposed to be closely related to a d-complex [ 141. This situation, in combination with the small value for the B-X transition frequency (see eq. (2 ) ),

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Table 1 Spectral line identifications and excitation wavenumbers (cm-‘), errors (one standard deviation) for the selected BaCl B %+ levels. (u’, v”)

total pressures

(Torr)

and measured

lifetimes

(ns) with statistical

v

P

7

fJ

(14)

P*(55)

12 106.070

0.11 1.0

103.0 94.5

2.5 1.8

(l>O)

Ri(79)

12105.541

0.19 0.82 1.8

102.3 98.3 97.9

0.8 0.7 0.5

(1,O)

Pr(40)

12105.957

0.22 0.80

98.2 98.3

2.5 1.5

(l>O)

P,(56)

and R,( 109) a)

12105.598

0.25 0.80 1.8

104.1 98.9 99.4

1.3 0.9 0.9

(130)

R,(61)

andP,(37)

12112.992

1.0

95.6

0.3

0.36 0.50 0.40

101.4 98.9 102.4

1.9 0.9 1.0

a)

(2>1)

Rr(59)

12087.496

(2,l)

Pz(37)

12087.248

a) Blended line.

would explain the relatively in the BaCl B *C state.

long lifetime

observed

4. Conclusion The power of low-cost GaAlAs-diode lasers in the field of time-resolved spectroscopy is evident from

the experiments presented in this paper. No other existing laser can compare with the engineering simplicity when high resolution is needed, both in the time domain and the wavelength domain. Lifetime measurements have previously been considered to belong to the expensive sciences. This is no longer

Table 2 Calculated Franck-Condon factors, r-centroids (A) and band oscillator strengths for the BaCl B ‘1+-X %+ transition. Molecular constants were taken from ref. [ 31.

+MHz

B-%+

v=o

v=l

v=2

v=3

0.747

0.22 1

2.70 0.075

2.79 0.022

0.029 2.88 0.003

0.002 2.97 0.0002

v=l

0.214 2.61 0.022

0.372 2.70 0.037

0.330 2.80 0.033

0.075 2.89 0.008

v=2

0.035

0.307

2.53 0.004

2.62 0.032

0.150 2.71 0.015

0.361 2.80 0.039

0.004

0.084

2.45 0.0004

2.54 0.009

0.320 2.63 0.033

0.038 2.72 0.004

v=o

I

I

0.5

I

1.0

I

1.5

p(Torrl

Fig. 5. Stern-Volmer plot of the reciprocal lifetime versus pressure with error bars (one standard deviation). Indicated in the figure are the linear and the parabolic fitted lines used for extrapolation to zero pressure.

X%+

v=3

117

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necessarily true, since an appropriate laser can be purchased for US $20. A limitation for the roomtemperature diode lasers has previously been the restriction of the wavelengths that could be reached using commercially available lasers. This limitation is gradually removed. Today the region 670-900 nm is covered in addition to the 1.3 urn and 1.55 ym spectral regions and work is in progress on the development of shorter wavelength diode lasers.

Acknowledgements We thank L. Klynning for stimulating discussions, and for the use of equipment. We also thank Hitachi Ltd. for supplying us with diode lasers. This work was supported by the Swedish National Research Council.

References [ I ] J.C. Camparo, Contemp. Phys. 26 ( 1985) 443. [ 21 T. Gustavsson and H. Martin, Rev. Sci. Instrum. 57 ( 1986) 1132.

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I5 June 1988

[ 31 T. Gustavsson and H. Martin, Physica Scripta 34 ( 1986) 207. [4] T. Gustavsson, H. Martin and P. Royen (submitted for publication). [ 51 B. Lindgren, H. Martin and U. Sassenberg, Laser spectroscopy VIII (Springer-Verlag, Berlin, 1987). [6] C.C. Jensen, T.G. Anderson, C. Reiser and J.I. Steinfeld, J. Chem. Phys. 71 (1979) 3648. [ 71 M. Harig, R. Charneau and H. Dubost, Phys. Rev. Lett. 49 (1982) 715. [8] P.J. Dagdigian, H.W. Cruse and R.N. Zare, J. Chem. Phys. 60 (1974) 2330. [9] SF. Rice, H. Martin and R.W. Field, J. Chem. Phys. 82 ( 1985) 5023. [lo] K. Sakurai, S.E. Johnson and H.P. Broida, J. Chem. Phys. 52 (1970) 1625. 111 L.-E. Berg, L. Klynning and H. Martin, Optics Comm. 17 (1976) 320. 121 L.-E. Berg, L. Klynning and H. Martin, Physica Scripta 2 1 (1980) 173. 131 MULPAC, University of Stockholm Institute of Physics Report 74-08, Stockholm 1974. [ 141 H. Martin and P. Royen, Chem. Phys. Lett. 97 (1983) 127. [ 151 W.R. Jairman and McCallum, Program TRAPRB Handbook CRESS York University, Ontario, Canada 197 1. [ 161 C. Ryzlewics, H.-U. Schutze-Pahlmann, J. Hoeft and T. Tarring, Chem. Phys. 71 (1982) 389. [ 171 A. Lindgird and S.E. Nielsen, At. Data Nucl. Data Tables 19 (1977) 533.