A. F. Ioffe Physicotechnical Institute, St. Peterburg, Russia Time-resolved study of the level-anticrossing effect in exciton emission A. S. Yakunenkov, A. N. Starukhin, D. K. Nelson, B. S. Razbirin

CONTENS The level-anticrossing effect. The anticrossing signal in optical emission spectra under the conditition of cw excitation. Problem definition. The modelling object for study – triplet bound excitons in GaSe. Experimental results. Interpretation of the results. Conclusion.

The level-anticrossing effect (H0 + Hmag) Y = EY {E} = E1, E2 H = H0 + Hmag+ V Ya = C1Y1 + C2Y2 Yb = C2Y1 - C1Y2

Anticrossing signal

Crystalline structure

Experimental set up Spectrometer Pump pulse Sample Cu-laser

Emission spectrum of GaSe crystal с Excitation

Energy level diagram of the triplet exciton in GaSe

Experimental anticrossing signal -exciton emission, I(B,t), measured at different times t during the excited state lifetime. The time t is specified in the figure. Thus, the experimental data demonstrate that the shape of the level-anticrossing signal measured at different moments within the bound exciton lifetime varies essentially from a practically complete absence of the signal to a complex structure with two maxima.

Zeeman effect diagram To interpret the observed evolution of the level-anticrossing signal, consider the energy level structure of bound exciton in GaSe.

Sublevel splitting diagram

Level-anticrossing signal The points are experimental data, and the solid lines are plots of theoretical relation r = 1.25107 s, 0 = 7106 s, ' = 0.0357 meV, 2V23 = 0.0045 meV

Theoretical diagram of emission components

Electronic band model of GaSe at 4.2K near Г and M points ≈

CONCLUSIONS The investigation of the level anticrossing effect in afterglow spectra reveals that the well-known shape of the anticrossing signal in the form of a simple maximum is only a particular case corresponding to the emission of a system at a certain time after the excitation. The signal profile may vary substantially with time, and it is possible to isolate the contributions to this signal due to different interacting states which cannot be discriminated spectrally in emission. An investigation of the level-anticrossing effect in afterglow spectra offers also, in principle, a possibility of obtaining information on the lifetimes of any one of the interacting states. The phenomenon observed should have a fairly general character and be observable in various atomic systems.

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Excitons (bound electron-hole pair)