Lifetime Measurements in Fast Beams

The most accurate method for lifetime measurements in the range of 10 -10 s is based on a modern version of an old technique that was used by W. Wien 70 years ago [799]. Here a time measurement is reduced to measurements of a pathlength and a velocity. 

The atomic or molecular ions produced in an ion source are accelerated by the voltage U and focused to form an ion beam. The different masses are separated by a magnet (Fig. 6.92) and the wanted ions are excited at the position x = 0 by a cw laser beam. The LIF is monitored as a function of the variable distance x between the excitation region and the position of a special photon detector mounted on a precision translational drive. Since the velocity v = leUlmf/ is known from the measured aeceleration voltage U, the time t = x/v s determined from the measured positions X. 

The excitation intensity can be increased if the excitation region is placed inside the resonator of a cw dye laser that is tuned to the selected transition. Before they reach the laser beam, the ions can be preexcited into highly excited long-living levels by gas collisions in a differentially pumped gas cell (Fig. 6.93a). This opens new transitions for the laser excitation and allows lifetime measurements of high-lying ionic states even with visible lasers [800]. 

Collisional preexcitation has the drawback that several levels are simultaneously excited, which may feed by cascading fluorescence transitions the level k) whose lifetime is to be measured. These cascades alter the time profile /pi(0 of the level A ) and falsify the real lifetime Xk (Fig. 6.93b). This problem can be solved by a special measurement cycle for each position x the fluorescence is measured alternately with and without laser excitation (Fig. 6.93c). The difference of both counting rates yields the LIF without cascade contributions. In order to eliminate fluctuations of the laser intensity or the ion beam intensity a second detector is installed at the fixed 

Lifetimes of atoms and ions have been measured very accurately with this technique. More experimental details and different versions of this laser beam method can be found in the extensive literature [800-804]. 

The atomic or molecular ions produced in an ion source are accelerated by the voltage U and focused to form an ion beam. The different masses are 

The time resolution At of the detectors is determined by their spatial resolution Ax and the velocity v of the ions or neutrals. In order to reach a good time resolution which is independent of the position x of the detector, one has to take care that the detector collects the fluorescence only from a small path interval Ax but still sees the whole cross section of the slightly divergent ion beam. This can be realized by specially designed bundles of optical fibers which are arranged in a conical circle around the beam axis 

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Fig. 6.94 Experimental arrangement for cascade-free lifetime measurements in fast beams of ions or neutrals with fluorescence collection by conically shaped optical-fiber bundles...

Fig.11.17. Schematic experimental arrangement for lifetime measurements in fast atomic or ion beams...

An electronic or vibrational excited state has a finite global lifetime and its de-excitation, when it is not metastable, is very fast compared to the standard measurement time conditions. Dedicated lifetime measurements are a part of spectroscopy known as time domain spectroscopy. One of the methods is based on the existence of pulsed lasers that can deliver radiation beams of very short duration and adjustable repetition rates. The frequency of the radiation pulse of these lasers, tuned to the frequency of a discrete transition, as in a free-electron laser (FEL), can be used to determine the lifetime of the excited state of the transition in a pump-probe experiment. In this method, a pump energy pulse produces a transient transmission dip of the sample at the transition frequency due to saturation. The evolution of this dip with time is probed by a low-intensity pulse at the same frequency, as a function of the delay between the pump and probe pulses.1 When the decay is exponential, the slope of the decay of the transmission dip as a function of the delay, plotted in a log-linear scale, provides a value of the lifetime of the excited state. 

Fig. 6.92 Lifetime measurements of highly excited levels of ions or neutral atoms and molecules in a fast beam...

The measurement of absolute beam velocities, or the calibration of voltages, is already quite sensitive to the relativistic quadratic term in the Doppler shift formula (7). In fact, this transverse Doppler shift, caused by the time dilatation factor y = (1 - j8 )" , was first observed in the spectral lines of fast-moving hydrogen atoms from a 30-keV beam of H2 ions, viewed along and opposite the direction of propagation. Comparable accuracy in the percent range was also achieved in Mossbauer experi-ments, and more recently the time dilatation factor on the muon lifetime was determined to 1 x 10". ... 

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