High-power impulse magnetron sputtering (HiPIMS) of materials systems with metal/gas-atom mass ratios m(Me)/m(g) near, or less than, unity presents a challenge for precise timing of synchronous substrate-bias pulses to select metal-ion irradiation of the film and, thus, reduce stress while increasing layer density during low-temperature growth. The problem stems from high gas-ion fluxes Fg+(t) at the substrate, which overlap with metal-ion fluxes FMe+(t). We use energy-and time-dependent mass spectrometry to analyze FMe+(t) and Fg+(t) for Group IVb transition-metal targets in Ar and show that the time-and energy-integrated metal/gas ion ratio NMe+/NAr+ at the substrate can be controlled over a wide range by adjusting the HiPIMS pulse length tau(ON), while maintaining the peak target current density J(T,peak) constant. The effect is a consequence of severe gas rarefaction which scales with J(T)(t). For Ti-HiPIMS, terminating the discharge at the maximum J(T)(t), corresponding to tau(ON) = 30 mu s, there is an essentially complete loss of Ar+ ion intensity, yielding NTi+/NAr+ similar to 60. With increasing tau(ON),J(T)(t) decreases and NTi+/NAr+ gradually decays, due to Ar refill, to similar to 1 with tau(ON) = 120 s. Time-resolved ion-energy distribution functions confirm that the degree of rarefaction depends on tau(ON): for shorter pulses, tau ONHTC/SUBTAG < FORTITLEHTC_RETAIN 60 [rs, the original sputtered-atom Sigmund-Thompson energy distributions are preserved long after the HiPIMS pulse, which is in distinct contrast to longer pulses, tau(ON) >= 60 mu s, for which the energy distributions collapse into narrow ther-malized peaks. Thus, optimizing the HiPIMS pulse width minimizes the gas-ion flux to the substrate independent of m(Me)/m(g).