  Based on Tunable Diode Laser Absorption Spectroscopy (TDLAS) measurements of
the spatially averaged and peak helium metastable atom densities in a
capacitively coupled micro atmospheric pressure plasma jet operated in He/N$_2$
mixtures, the energy efficiency of metastable species (He-I 2$^3$S$_1$)
generation is compared for three different scenarios: single frequency
operation at (i) 13.56 MHz and (ii) 54.12 MHz, and Voltage Waveform Tailoring
(VWT) at (iii) "valleys"-waveforms synthesized from four consecutive harmonics
of 13.56 MHz. For each case, the dissipated power is measured based on a
careful calibration procedure of voltage and current measurements. The results
are compared to PIC/MCC simulation results and very good agreement is found.
The computational results show that the choice of the surface coefficients in
the simulation is important to reproduce the experimental data correctly. Due
to the enhanced control of the spatio-temporal electron power absorption
dynamics and, thus, of the electron energy distribution function by VWT, this
approach does not only provide better control of the generation of excited and
reactive species compared to single frequency excitation, but in case of helium
metastables the energy efficiency is also shown to be significantly higher in
case of VWT.
