We have studied the effects of the plasma density scale length on the absorption mechanism of the femtosecond (fs) laser pulses interacting with solid targets. Experiments and particle-in-cell (PIC) simulations demonstrate that the vacuum heating is the main absorption in the plasma in the interaction of fs laser pulses with solid targets when no prepulses are applied. The energy spectrum of hot electrons ejected out of or injected into the plasma show a bitemperature distribution. While the first temperature of the two groups of hot electrons can be attributed to the "pull-and-push" exertion of the laser field, the second temperature refers to the electrons accelerated by the static part (in front of the target) and the oscillating part (in the plasma layer) of the laser-induced electric field, respectively. PIC simulations also show that with an appropriate density scale length, the femtosecond laser energy can be absorbed locally through different mechanisms.