Observations of stimulated electromagnetic emission (SEE) around the second harmonic of Tromsø heater frequency carried out by long-distance diagnostic HF tools are presented. Diagnostic tools are located at a distance of 1200 km from the Tromsø HF heater. Observed SEE spectra show: (1) spectral broadening up to 30 kHz; (2) frequency shift of the maximum amplitude in the SEE spectra from the heater second harmonic frequency 8080 kHz by 16–20 kHz; (3) dependence of the maximum amplitude on the heater power level. The detected features in the SEE spectra have not been observed before in ionospheric heating experiments and may be the second harmonic of the downshifted maximum. The strongest candidate is the coalescense (head-on collisions) of Langmuir waves produced in the heated volume.